configure: check realloc with AC_CHECK_FUNCS() to fix cross-compilation.

The [AC_PROG_CC_C99 macro](https://www.gnu.org/software/autoconf/manual/autoconf-2.64/html_node/C-Compiler.html) sets the complier to the C99 standard if the compiler does not default to such behaviour (albeit gnu99). This is better than specifically hardcoding flags

AC_FUNC_REALLOC is messed up when cross-compiling, ending up in failed
build with "undefined reference to `rpl_realloc'" error.
AC_CHECK_FUNCS will work both with native and cross builds.

- ./configure would fail due to bad code generation

Link against libm when needed

In certain C libraries (e.g uClibc), isnan() and related functions are
implemented in libm, so json-c needs to link against it. This commit
therefore adds an AC_TRY_LINK() test to check whether a program
calling isnan() can be properly linked with no special flags. If not,
we assume linking against libm is needed.

The json-c.pc.in file is also adjusted so that in the case of static
linking against json-c, -lm is also used.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

* CVE-2013-6371: hash collision denial of service
* CVE-2013-6370: buffer overflow if size_t is larger than int

Conflicts:
	Makefile.am