CMake was not properly substituting the installation dir variables (they
ended up all empty), so the pkg-config results were also wrongs. For
instance cflags was: -I -I/json-c
Even though json-c was found at configure time, this obviously broke the
build of any application using it.
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.
Jehan