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

Nzbuu/json-c,Nzbuu/json-c,Nzbuu/json-c,Nzbuu/json-c