Fix most -Wundef warnings

In some cases #if was used instead of #ifdef in an inconsistent style.
Cleaning this up also helps when analyzing cases like
38d8dce61f where this makes a
difference.

There are no behavior changes here, but the change in pg_bswap.h would
prevent possible accidental misuse by third-party code.

Discussion: https://www.postgresql.org/message-id/flat/3b615ca5-c595-3f1d-fdf7-a429e564f614%402ndquadrant.com

src/port/snprintf.c

@@ -1052,7 +1052,7 @@ fmtint(long long value, char type, int forcesign, int leftjust,
 	}
 
 	/* disable MSVC warning about applying unary minus to an unsigned value */
-#if _MSC_VER
+#ifdef _MSC_VER
 #pragma warning(push)
 #pragma warning(disable: 4146)
 #endif
@@ -1061,7 +1061,7 @@ fmtint(long long value, char type, int forcesign, int leftjust,
 		uvalue = -(unsigned long long) value;
 	else
 		uvalue = (unsigned long long) value;
-#if _MSC_VER
+#ifdef _MSC_VER
 #pragma warning(pop)
 #endif
 

src/port/win32error.c

@@ -188,7 +188,7 @@ _dosmaperr(unsigned long e)
 			ereport(DEBUG5,
 					(errmsg_internal("mapped win32 error code %lu to %d",
 									 e, doserr)));
-#elif FRONTEND_DEBUG
+#elif defined(FRONTEND_DEBUG)
 			fprintf(stderr, "mapped win32 error code %lu to %d", e, doserr);
 #endif
 			errno = doserr;