Require a C99 compiler.

We removed this in 1.2 because it was too early for dependencies such as OMPI.
OMPI has been enforcing C99 for several years now, and basic features
(dynamic arrays n the stack, designated initializers, etc) work fine.

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

NEWS

@@ -127,6 +127,7 @@ Version 2.0.0
   + HWLOC_COMPONENTS takes precedence over other environment variables
     for selecting components.
   + Remove the dependency on libnuma on Linux.
+  + hwloc now requires a C99 compliant compiler.
 
 
 Version 1.11.8

config/hwloc.m4

@@ -12,6 +12,7 @@ dnl                         University of Stuttgart.  All rights reserved.
 dnl Copyright © 2006-2017 Cisco Systems, Inc.  All rights reserved.
 dnl Copyright © 2012  Blue Brain Project, BBP/EPFL. All rights reserved.
 dnl Copyright © 2012       Oracle and/or its affiliates.  All rights reserved.
+dnl Copyright © 2012  Los Alamos National Security, LLC. All rights reserved.
 dnl See COPYING in top-level directory.
 
 # Main hwloc m4 macro, to be invoked by the user
@@ -150,6 +151,14 @@ EOF])
           [AC_DEFINE([HWLOC_SYM_TRANSFORM], [0])],
           [AC_DEFINE([HWLOC_SYM_TRANSFORM], [1])])
 
+    # hwloc 2.0+ requires a C99 compliant compiler
+    AC_PROG_CC_C99
+    # The result of AC_PROG_CC_C99 is stored in ac_cv_prog_cc_c99
+    if test "x$ac_cv_prog_cc_c99" = xno ; then
+        AC_MSG_WARN([hwloc requires a C99 compiler])
+        AC_MSG_ERROR([Aborting.])
+    fi
+
     # GCC specifics.
     if test "x$GCC" = "xyes"; then
         HWLOC_GCC_CFLAGS="-Wall -Wmissing-prototypes -Wundef"