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

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.ac

@@ -43,7 +43,7 @@ AC_TYPE_SIZE_T
 # Checks for library functions.
 AC_FUNC_VPRINTF
 AC_FUNC_MEMCMP
-AC_FUNC_REALLOC
+AC_CHECK_FUNCS([realloc])
 AC_CHECK_FUNCS(strcasecmp strdup strerror snprintf vsnprintf vasprintf open vsyslog strncasecmp setlocale)
 AC_CHECK_DECLS([INFINITY], [], [], [[#include <math.h>]])
 AC_CHECK_DECLS([nan], [], [], [[#include <math.h>]])