Define _POSIX_C_SOURCE to be 200112L, as a minimum for C99.

Strict platforms cannot be expected to accept C99 code as valid
when earlier standards versions are selected.

This helps the programs build on Solaris-like platforms (e.g.
illumos).

Fixes #3420

Signed-off-by: nia <nia@netbsd.org>

tests/suites/main_test.function

diff --git a/tests/suites/main_test.function b/tests/suites/main_test.function
index ff4cf20..e56191a 100644
--- a/tests/suites/main_test.function
+++ b/tests/suites/main_test.function
@@ -21,7 +21,7 @@
 
 #if defined(__unix__) || (defined(__APPLE__) && defined(__MACH__))
 #if !defined(_POSIX_C_SOURCE)
-#define _POSIX_C_SOURCE 1 // for fileno() from <stdio.h>
+#define _POSIX_C_SOURCE 200112L // for fileno() from <stdio.h>
 #endif
 #endif