Move NON_EXEC_STATIC from c.h to postgres.h

It is not needed at the scope of c.h, only in backend code.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/a6a6b48e-ca0a-b58d-18de-98e40d94b842%40enterprisedb.com

src/include/c.h

@@ -1327,13 +1327,6 @@ typedef intptr_t sigjmp_buf[5];
 #endif							/* __MINGW64__ */
 #endif							/* WIN32 */
 
-/* EXEC_BACKEND defines */
-#ifdef EXEC_BACKEND
-#define NON_EXEC_STATIC
-#else
-#define NON_EXEC_STATIC static
-#endif
-
 /* /port compatibility functions */
 #include "port.h"
 

src/include/postgres.h

@@ -25,6 +25,7 @@
  *	  -------	------------------------------------------------
  *		1)		variable-length datatypes (TOAST support)
  *		2)		Datum type + support macros
+ *		3)		miscellaneous
  *
  *	 NOTES
  *
@@ -805,4 +806,23 @@ extern Datum Float8GetDatum(float8 X);
 #define Float8GetDatumFast(X) PointerGetDatum(&(X))
 #endif
 
+
+/* ----------------------------------------------------------------
+ *				Section 3:	miscellaneous
+ * ----------------------------------------------------------------
+ */
+
+/*
+ * NON_EXEC_STATIC: It's sometimes useful to define a variable or function
+ * that is normally static but extern when using EXEC_BACKEND (see
+ * pg_config_manual.h).  There would then typically be some code in
+ * postmaster.c that uses those extern symbols to transfer state between
+ * processes or do whatever other things it needs to do in EXEC_BACKEND mode.
+ */
+#ifdef EXEC_BACKEND
+#define NON_EXEC_STATIC
+#else
+#define NON_EXEC_STATIC static
+#endif
+
 #endif							/* POSTGRES_H */