perl: Hide warnings inside perl.h when using gcc compatible compiler

New versions of perl trigger warnings within perl.h with our compiler
flags. At least -Wdeclaration-after-statement, -Wshadow=compatible-local are
known to be problematic.

To avoid these warnings, conditionally use #pragma GCC system_header before
including plperl.h.

Alternatively, we could add the include paths for problematic headers with
-isystem, but that is a larger hammer and is harder to search for.

A more granular alternative would be to use #pragma GCC diagnostic
push/ignored/pop, but gcc warns about unknown warnings being ignored, so every
to-be-ignored-temporarily compiler warning would require its own pg_config.h
symbol and #ifdef.

As the warnings are voluminous, it makes sense to backpatch this change. But
don't do so yet, we first want gather buildfarm coverage - it's e.g. possible
that some compiler claiming to be gcc compatible has issues with the pragma.

Author: Andres Freund <andres@anarazel.de>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: Discussion: https://postgr.es/m/20221228182455.hfdwd22zztvkojy2@awork3.anarazel.de

src/include/c.h

@@ -298,6 +298,34 @@
 #endif
 #endif
 
+/*
+ * Does the compiler support #pragma GCC system_header? We optionally use it
+ * to avoid warnings that we can't fix (e.g. in the perl headers).
+ * See https://gcc.gnu.org/onlinedocs/cpp/System-Headers.html
+ *
+ * Headers for which we do not want to show compiler warnings can,
+ * conditionally, use #pragma GCC system_header to avoid warnings. Obviously
+ * this should only be used for external headers over which we do not have
+ * control.
+ *
+ * Support for the pragma is tested here, instead of during configure, as gcc
+ * also warns about the pragma being used in a .c file. It's surprisingly hard
+ * to get autoconf to use .h as the file-ending. Looks like gcc has
+ * implemented the pragma since the 2000, so this test should suffice.
+ *
+ *
+ * Alternatively, we could add the include paths for problematic headers with
+ * -isystem, but that is a larger hammer and is harder to search for.
+ *
+ * A more granular alternative would be to use #pragma GCC diagnostic
+ * push/ignored/pop, but gcc warns about unknown warnings being ignored, so
+ * every to-be-ignored-temporarily compiler warning would require its own
+ * pg_config.h symbol and #ifdef.
+ */
+#ifdef __GNUC__
+#define HAVE_PRAGMA_GCC_SYSTEM_HEADER	1
+#endif
+
 
 /* ----------------------------------------------------------------
  *				Section 2:	bool, true, false

src/pl/plperl/plperl.h

@@ -70,6 +70,15 @@
 #define HAS_BOOL 1
 #endif
 
+/*
+ * Newer versions of the perl headers trigger a lot of warnings with our
+ * compiler flags (at least -Wdeclaration-after-statement,
+ * -Wshadow=compatible-local are known to be problematic). The system_header
+ * pragma hides warnings from within the rest of this file, if supported.
+ */
+#ifdef HAVE_PRAGMA_GCC_SYSTEM_HEADER
+#pragma GCC system_header
+#endif
 
 /*
  * Get the basic Perl API.  We use PERL_NO_GET_CONTEXT mode so that our code