Add declaration-level assertions for compile-time checks

Those new assertions can be used at file scope, outside of any function
for compilation checks.  This commit provides implementations for C and
C++, and fallback implementations.

Author: Peter Smith
Reviewed-by: Andres Freund, Kyotaro Horiguchi, Dagfinn Ilmari Mannsåker,
Michael Paquier
Discussion: https://postgr.es/m/201DD0641B056142AC8C6645EC1B5F62014B8E8030@SYD1217

src/common/relpath.c

@@ -37,6 +37,9 @@ const char *const forkNames[] = {
 	"init"						/* INIT_FORKNUM */
 };
 
+StaticAssertDecl(lengthof(forkNames) == (MAX_FORKNUM + 1),
+				 "array length mismatch");
+
 /*
  * forkname_to_number - look up fork number by name
  *