Autoconf: detect and set CFLAGS/CPPFLAGS needed for C99 mode

This is the first phase of addressing #11757 which aims to make C99
support a base-line requirement for GHC and clean up the code-base to
use C99 facilities when sensible.

This patch exploits the logic/heuristic used by `AC_PROG_CC_C99` to
determine the flags needed in case the C compiler isn't able to compile
C99 code in its current mode. We can't use `AC_PROG_CC_C99` directly
though because GHC's build-system expects CC to contain a filename
without any flags, while `AC_PROG_CC_C99` would e.g. result in
`CC="gcc -std=gnu99"`. Morever, we support different `CC`s for
stage0/1/2, so we need a version of `AC_PROG_CC_C99` for which we can
specify the `CC`/`CFLAGS` variables to operate on. This is what
`FP_SET_CFLAGS_C99` does.

Note that Clang has been defaulting to C99+ for a long time, while GCC 5
defaults to C99+ as well. So this has mostly an affect on older GCCs
versions prior to 5.0 and possibly compilers other than GCC/Clang (which
are not officially supported for building GHC anyway).

Reviewers: kgardas, erikd, bgamari, austin

Reviewed By: erikd

Differential Revision: https://phabricator.haskell.org/D2045

1 files changed, 4 insertions, 0 deletions

diff --git a/includes/Stg.h b/includes/Stg.h
index b179db6deb..f1949b1630 100644
--- a/includes/Stg.h
+++ b/includes/Stg.h
@@ -26,6 +26,10 @@
 #ifndef STG_H
 #define STG_H
 
+#if !(__STDC_VERSION__ >= 199901L)
+# error __STDC_VERSION__ does not advertise C99 or later
+#endif
+
 /*
  * If we are compiling a .hc file, then we want all the register
  * variables.  This is the what happens if you #include "Stg.h" first: