build: fix FDELT_TYPE configure check

This probably never worked properly. Confirmed working now with every compiler
I throw at it.

configure.ac

@@ -332,7 +332,10 @@ if test x$use_glibc_compat != xno; then
   #__fdelt_chk's params and return type have changed from long unsigned int to long int.
   # See which one is present here.
   AC_MSG_CHECKING(__fdelt_chk type)
-  AC_TRY_COMPILE([#define __USE_FORTIFY_LEVEL 2
+  AC_TRY_COMPILE([#ifdef _FORTIFY_SOURCE
+                    #undef _FORTIFY_SOURCE
+                  #endif
+                  #define _FORTIFY_SOURCE 2
                   #include <sys/select.h>
      extern "C" long unsigned int __fdelt_warn(long unsigned int);],[],
     [ fdelt_type="long unsigned int"],