If configure script is executed with stricter cflags
"-Werrorr=implicit-function-declaration -Werror=implicit-int"
then detection of few features will fail.

 Checking for C99 vsnprintf : not found
 Checking for HAVE_SHARED_MMAP : not found
 Checking for HAVE_MREMAP : not found

lib/replace/test/shared_mmap.c:18:1:
    error: return type defaults to ‘int’ [-Werror=implicit-int]
 main()
 ^~~~
lib/replace/test/shared_mmap.c: In function ‘main’:
lib/replace/test/shared_mmap.c:25:16:
    error: implicit declaration of function ‘exit’
    [-Werror=implicit-function-declaration]
  if (fd == -1) exit(1);
                ^~~~
lib/replace/test/shared_mmap.c:25:16:
    warning: incompatible implicit declaration of built-in function ‘exit’
lib/replace/test/shared_mmap.c:25:16:
    note: include ‘<stdlib.h>’ or provide a declaration of ‘exit’

Signed-off-by: Lukas Slebodnik <lslebodn@redhat.com>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>

use CHECK_CODE()