Add static assert on IS_ENABLED() macro

This patch adds a build-time oracle for IS_ENABLED(). MSVC's legacy
preprocessor mis-handles the token-paste + rescan and silently returns 0
for a defined-as-1 flag, disabling every IS_ENABLED-gated feature.

Related-to: goToMain/libosdp#289
Signed-off-by: Siddharth Chandrasekaran <sidcha.dev@gmail.com>

Author: sidcha

src/utils.c

@@ -12,6 +12,13 @@
 
 #include <utils/utils.h>
 
+#define _IS_ENABLED_SELFTEST_ON 1
+_Static_assert(IS_ENABLED(_IS_ENABLED_SELFTEST_ON) == 1,
+	       "IS_ENABLED(): defined-as-1 flag must evaluate to 1");
+_Static_assert(IS_ENABLED(_IS_ENABLED_SELFTEST_OFF) == 0,
+	       "IS_ENABLED(): undefined flag must evaluate to 0");
+#undef _IS_ENABLED_SELFTEST_ON
+
 int randint(int limit)
 {
 	int r;