Handle g++ as a compiler when -Wstrict-prototypes is an invalid option, this...

Handle g++ as a compiler when -Wstrict-prototypes is an invalid option, this emits a warning which can become an error.

To add to the fun the standard test program uses 'int main()', so we need our own program to check things, also PACKAGE_URL is a macro re-define, also an error...

configure.ac

@@ -19,9 +19,6 @@
 AC_INIT([SWIFT],[0.7.0],[https://gitlab.cosma.dur.ac.uk/swift/swiftsim])
 swift_config_flags="$*"
 
-#  Need to define this, instead of using fifth argument of AC_INIT, until 2.64.
-AC_DEFINE([PACKAGE_URL],["www.swiftsim.com"], [Package web pages])
-
 AC_COPYRIGHT
 AC_CONFIG_SRCDIR([src/space.c])
 AC_CONFIG_AUX_DIR([.])
@@ -687,7 +684,7 @@ if test "x$with_profiler" != "xno"; then
       proflibs="-lprofiler"
    fi
    AC_CHECK_LIB([profiler],[ProfilerFlush],
-    [have_profiler="yes" 
+    [have_profiler="yes"
       AC_DEFINE([WITH_PROFILER],1,[Link against the gperftools profiling library.])],
     [have_profiler="no"], $proflibs)
 
@@ -830,16 +827,17 @@ if test "$enable_warn" != "no"; then
     # We will do this by hand instead and only default to the macro for unknown compilers
     case "$ax_cv_c_compiler_vendor" in
           gnu | clang)
-             CFLAGS="$CFLAGS -Wall -Wextra -Wno-unused-parameter -Wshadow -Wstrict-prototypes"
+             CFLAGS="$CFLAGS -Wall -Wextra -Wno-unused-parameter -Wshadow"
           ;;
 	  intel)
-             CFLAGS="$CFLAGS -w2 -Wunused-variable -Wshadow -Wstrict-prototypes"
+             CFLAGS="$CFLAGS -w2 -Wunused-variable -Wshadow"
           ;;
 	  *)
 	     AX_CFLAGS_WARN_ALL
 	  ;;
     esac
 
+
     # Add a "choke on warning" flag if it exists
     if test "$enable_warn" = "error"; then
        case "$ax_cv_c_compiler_vendor" in
@@ -848,6 +846,11 @@ if test "$enable_warn" != "no"; then
           ;;
        esac
     fi
+
+    # We want strict-prototypes, but this must still work even if warnings
+    # are an error.
+    AX_CHECK_COMPILE_FLAG([-Wstrict-prototypes],[CFLAGS="$CFLAGS -Wstrict-prototypes"],
+                          [CFLAGS="$CFLAGS"],[$CFLAGS],[AC_LANG_SOURCE([int main(void){return 0;}])])
 fi
 
 # Various package configuration options.
@@ -1211,6 +1214,11 @@ AC_DEFINE_UNQUOTED([SWIFT_CONFIG_FLAGS],["$swift_config_flags"],[Flags passed to
 # Make sure the latest git revision string gets included
 touch src/version.c
 
+#  Need to define this, instead of using fifth argument of AC_INIT, until
+#  2.64. Defer until now as this redefines PACKAGE_URL, which can emit a
+#  compilation error when testing with -Werror.
+AC_DEFINE([PACKAGE_URL],["www.swiftsim.com"], [Package web pages])
+
 # Generate output.
 AC_OUTPUT