common: fix compilation when strnlen, strnup and memrchr are not available

common: fix compilation when strnlen, strnup and memrchr are not available

Add a few casts to fix build failure when these functions are not
available.  For example:

      CXX      lttng-conf.o
In file included from /home/smarchi/src/lttng-tools/src/common/macros.h:15:0,
                 from /home/smarchi/src/lttng-tools/src/common/error.h:19,
                 from /home/smarchi/src/lttng-tools/src/common/common.h:12,
                 from /home/smarchi/src/lttng-tools/src/bin/lttng/conf.cpp:18:
/home/smarchi/src/lttng-tools/src/common/compat/string.h: In function ‘size_t lttng_strnlen(const char*, size_t)’:
/home/smarchi/src/lttng-tools/src/common/compat/string.h:28:14: error: invalid conversion from ‘const void*’ to ‘const char*’ [-fpermissive]
  end = memchr(str, 0, max);
        ~~~~~~^~~~~~~~~~~~~

This can be tested by modifying config.cache and setting the relevant
config variables to "no".

Change-Id: Ieb6debf32c82927767ad32d2f4aa62fb829a9f9f
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>