Create libiberty.m4, have GDB and GDBserver use it

Create libiberty.m4, have GDB and GDBserver use it

Converting GDB to be a C++ program, I stumbled on 'basename' issues,
like:

 src/gdb/../include/ansidecl.h:169:64: error: new declaration ‘char* basename(const char*)’
 /usr/include/string.h:597:26: error: ambiguates old declaration ‘const char* basename(const char*)’

which I believe led to this bit in gold's configure.ac:

 dnl We have to check these in C, not C++, because autoconf generates
 dnl tests which have no type information, and current glibc provides
 dnl multiple declarations of functions like basename when compiling
 dnl with C++.
 AC_CHECK_DECLS([basename, ffs, asprintf, vasprintf, snprintf, vsnprintf, strverscmp])

These checks IIUC intend to generate all the HAVE_DECL_FOO symbols
that libiberty.h and ansidecl.h check.

GDB is missing these checks currently, which results in the conflict
shown above.

This adds an m4 file that both GDB and GDBserver's configury use to
pull in the autoconf checks that libiberty clients needs done in order
to use these libiberty.h/ansidecl.h.

gdb/ChangeLog:
2015-02-27  Pedro Alves  <palves@redhat.com>

	* libiberty.m4: New file.
	* acinclude.m4: Include libiberty.m4.
	* configure.ac: Call libiberty_INIT.
	* config.in, configure: Regenerate.

gdb/gdbserver/
2015-02-27  Pedro Alves  <palves@redhat.com>

	* acinclude.m4: Include libiberty.m4.
	* configure.ac: Call libiberty_INIT.
	* config.in, configure: Regenerate.