build: Fix implicit declaration of function errors

Include the correct system headers for each test that meson performs.
This allows system capabilities to be detected correctly even if
implicit declaration of functions is considered an error.

meson.build

@@ -962,6 +962,7 @@ else
   vsnprintf_c99_test_code = '''
 #include <stdio.h>
 #include <stdarg.h>
+#include <stdlib.h>
 
 int
 doit(char * s, ...)
@@ -1011,6 +1012,7 @@ main(void)
   snprintf_c99_test_code = '''
 #include <stdio.h>
 #include <stdarg.h>
+#include <stdlib.h>
 
 int
 doit()
@@ -1057,6 +1059,8 @@ main(void)
 
   printf_unix98_test_code = '''
 #include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
 
 int
 main (void)
@@ -2174,6 +2178,7 @@ cmdline_test_code = '''
 #include <sys/stat.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <unistd.h>
 
 static int
 __getcmdline (void)