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diff --git a/m4/ax_compiler_flags_cflags.m4 b/m4/ax_compiler_flags_cflags.m4
new file mode 100644
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+++ b/m4/ax_compiler_flags_cflags.m4
@@ -0,0 +1,161 @@
+# =============================================================================
+#  https://www.gnu.org/software/autoconf-archive/ax_compiler_flags_cflags.html
+# =============================================================================
+#
+# SYNOPSIS
+#
+#   AX_COMPILER_FLAGS_CFLAGS([VARIABLE], [IS-RELEASE], [EXTRA-BASE-FLAGS], [EXTRA-YES-FLAGS])
+#
+# DESCRIPTION
+#
+#   Add warning flags for the C compiler to VARIABLE, which defaults to
+#   WARN_CFLAGS.  VARIABLE is AC_SUBST-ed by this macro, but must be
+#   manually added to the CFLAGS variable for each target in the code base.
+#
+#   This macro depends on the environment set up by AX_COMPILER_FLAGS.
+#   Specifically, it uses the value of $ax_enable_compile_warnings to decide
+#   which flags to enable.
+#
+# LICENSE
+#
+#   Copyright (c) 2014, 2015 Philip Withnall <philip@tecnocode.co.uk>
+#   Copyright (c) 2017, 2018 Reini Urban <rurban@cpan.org>
+#
+#   Copying and distribution of this file, with or without modification, are
+#   permitted in any medium without royalty provided the copyright notice
+#   and this notice are preserved.  This file is offered as-is, without any
+#   warranty.
+
+#serial 17
+
+AC_DEFUN([AX_COMPILER_FLAGS_CFLAGS],[
+    AC_REQUIRE([AC_PROG_SED])
+    AX_REQUIRE_DEFINED([AX_APPEND_COMPILE_FLAGS])
+    AX_REQUIRE_DEFINED([AX_APPEND_FLAG])
+    AX_REQUIRE_DEFINED([AX_CHECK_COMPILE_FLAG])
+
+    # Variable names
+    m4_define([ax_warn_cflags_variable],
+              [m4_normalize(ifelse([$1],,[WARN_CFLAGS],[$1]))])
+
+    AC_LANG_PUSH([C])
+
+    AC_COMPILE_IFELSE([AC_LANG_PROGRAM([
+      [#ifndef __cplusplus
+       #error "no C++"
+       #endif]])],
+      [ax_compiler_cxx=yes;],
+      [ax_compiler_cxx=no;])
+
+    # Always pass -Werror=unknown-warning-option to get Clang to fail on bad
+    # flags, otherwise they are always appended to the warn_cflags variable, and
+    # Clang warns on them for every compilation unit.
+    # If this is passed to GCC, it will explode, so the flag must be enabled
+    # conditionally.
+    AX_CHECK_COMPILE_FLAG([-Werror=unknown-warning-option],[
+        ax_compiler_flags_test="-Werror=unknown-warning-option"
+    ],[
+        ax_compiler_flags_test=""
+    ])
+
+    # Check that -Wno-suggest-attribute=format is supported
+    AX_CHECK_COMPILE_FLAG([-Wno-suggest-attribute=format],[
+        ax_compiler_no_suggest_attribute_flags="-Wno-suggest-attribute=format"
+    ],[
+        ax_compiler_no_suggest_attribute_flags=""
+    ])
+
+    # Base flags
+    AX_APPEND_COMPILE_FLAGS([ dnl
+        -fno-strict-aliasing dnl
+        $3 dnl
+    ],ax_warn_cflags_variable,[$ax_compiler_flags_test])
+
+    AS_IF([test "$ax_enable_compile_warnings" != "no"],[
+        if test "$ax_compiler_cxx" = "no" ; then
+            # C-only flags. Warn in C++
+            AX_APPEND_COMPILE_FLAGS([ dnl
+                -Wnested-externs dnl
+                -Wmissing-prototypes dnl
+                -Wstrict-prototypes dnl
+                -Wdeclaration-after-statement dnl
+                -Wimplicit-function-declaration dnl
+                -Wold-style-definition dnl
+                -Wjump-misses-init dnl
+            ],ax_warn_cflags_variable,[$ax_compiler_flags_test])
+        fi
+
+        # "yes" flags
+        AX_APPEND_COMPILE_FLAGS([ dnl
+            -Wall dnl
+            -Wextra dnl
+            -Wundef dnl
+            -Wwrite-strings dnl
+            -Wpointer-arith dnl
+            -Wmissing-declarations dnl
+            -Wredundant-decls dnl
+            -Wno-unused-parameter dnl
+            -Wno-missing-field-initializers dnl
+            -Wformat=2 dnl
+            -Wcast-align dnl
+            -Wformat-nonliteral dnl
+            -Wformat-security dnl
+            -Wsign-compare dnl
+            -Wstrict-aliasing dnl
+            -Wshadow dnl
+            -Winline dnl
+            -Wpacked dnl
+            -Wmissing-format-attribute dnl
+            -Wmissing-noreturn dnl
+            -Winit-self dnl
+            -Wredundant-decls dnl
+            -Wmissing-include-dirs dnl
+            -Wunused-but-set-variable dnl
+            -Warray-bounds dnl
+            -Wreturn-type dnl
+            -Wswitch-enum dnl
+            -Wswitch-default dnl
+            -Wduplicated-cond dnl
+            -Wduplicated-branches dnl
+            -Wlogical-op dnl
+            -Wrestrict dnl
+            -Wnull-dereference dnl
+            -Wdouble-promotion dnl
+            $4 dnl
+            $5 dnl
+            $6 dnl
+            $7 dnl
+        ],ax_warn_cflags_variable,[$ax_compiler_flags_test])
+    ])
+    AS_IF([test "$ax_enable_compile_warnings" = "error"],[
+        # "error" flags; -Werror has to be appended unconditionally because
+        # it's not possible to test for
+        #
+        # suggest-attribute=format is disabled because it gives too many false
+        # positives
+        AX_APPEND_FLAG([-Werror],ax_warn_cflags_variable)
+
+        AX_APPEND_COMPILE_FLAGS([ dnl
+            [$ax_compiler_no_suggest_attribute_flags] dnl
+        ],ax_warn_cflags_variable,[$ax_compiler_flags_test])
+    ])
+
+    # In the flags below, when disabling specific flags, always add *both*
+    # -Wno-foo and -Wno-error=foo. This fixes the situation where (for example)
+    # we enable -Werror, disable a flag, and a build bot passes CFLAGS=-Wall,
+    # which effectively turns that flag back on again as an error.
+    for flag in $ax_warn_cflags_variable; do
+        AS_CASE([$flag],
+                [-Wno-*=*],[],
+                [-Wno-*],[
+                    AX_APPEND_COMPILE_FLAGS([-Wno-error=$(AS_ECHO([$flag]) | $SED 's/^-Wno-//')],
+                                            ax_warn_cflags_variable,
+                                            [$ax_compiler_flags_test])
+                ])
+    done
+
+    AC_LANG_POP([C])
+
+    # Substitute the variables
+    AC_SUBST(ax_warn_cflags_variable)
+])dnl AX_COMPILER_FLAGS