Add pg_noinline macro to c.h.

Forcing a function not to be inlined can be useful if it's the
slow-path of a performance critical function, or should be visible in
profiles to allow for proper cost attribution.

Author: Andres Freund
Discussion: https://postgr.es/m/20170914061207.zxotvyopetm7lrrp@alap3.anarazel.de

src/include/c.h

@@ -642,6 +642,22 @@ typedef NameData *Name;
 #define pg_attribute_noreturn()
 #endif
 
+
+/*
+ * Forcing a function not to be inlined can be useful if it's the slow-path of
+ * a performance critical function, or should be visible in profiles to allow
+ * for proper cost attribution.
+ */
+/* GCC, Sunpro and XLC support noinline via __attribute */
+#if defined(__GNUC__) || defined(__SUNPRO_C) || defined(__IBMC__)
+#define pg_noinline __attribute__((noinline))
+/* msvc via declspec */
+#elif defined(_MSC_VER)
+#define pg_noinline __declspec(noinline)
+#else
+#define pg_noinline
+#endif
+
 /* ----------------------------------------------------------------
  *				Section 6:	assertions
  * ----------------------------------------------------------------