build: remove stdlib.h from header checks

We already use a mix of <cstdlib> and stdlib.h unconditionally throughout
the codebase.

Us checking this header also duplicates work already done by autotools.
Currently stdlib.h is checked for 3 times during a ./configure run, after
this change, at least it's only twice.

configure.ac

@@ -1010,7 +1010,7 @@ if test "$TARGET_OS" = "darwin"; then
   AX_CHECK_LINK_FLAG([-Wl,-bind_at_load], [HARDENED_LDFLAGS="$HARDENED_LDFLAGS -Wl,-bind_at_load"], [], [$LDFLAG_WERROR])
 fi
 
-AC_CHECK_HEADERS([endian.h sys/endian.h byteswap.h stdlib.h unistd.h sys/types.h sys/stat.h sys/select.h sys/prctl.h sys/sysctl.h vm/vm_param.h sys/vmmeter.h sys/resources.h])
+AC_CHECK_HEADERS([endian.h sys/endian.h byteswap.h unistd.h sys/types.h sys/stat.h sys/select.h sys/prctl.h sys/sysctl.h vm/vm_param.h sys/vmmeter.h sys/resources.h])
 
 AC_CHECK_DECLS([getifaddrs, freeifaddrs],[CHECK_SOCKET],,
     [#include <sys/types.h>