build: Modify maintainer compiler flags values

We use the micro version for distinguishing released tarballs and Git
builds; the maintainer compiler flags should be enabled for the latter,
and not just for unstable cycles, since it makes sense to have extra
warning flags even on stable cycles.

We also want to allow people to turn on -Werror on demand, so let's add
a third option to --enable-maintainer-flags.

README

@@ -148,9 +148,10 @@ Clutter has additional command line options for the configure script:
         no:
                 Disable support for COGL runtime debugging notes.
 
- --enable-maintainer-flags=[no/yes]
+ --enable-maintainer-flags=[no/yes/error]
         Use strict compiler flags. This defaults to 'yes' for developers
-        snapshots and to 'no' for stable releases.
+        snapshots and to 'no' for stable releases. If 'error' is used, then
+        -Werror will be enabled (if available).
 
  --enable-gtk-doc
 	use gtk-doc to build API documentation (default=no). Requires gtk-doc

configure.ac

@@ -1061,22 +1061,38 @@ AC_SUBST([GCOV_LDFLAGS])
 
 dnl === Enable strict compiler flags ==========================================
 
-# use strict compiler flags only on development releases
-m4_define([maintainer_flags_default], [m4_if(m4_eval(clutter_minor_version % 2), [1], [yes], [no])])
+# use strict compiler flags only when building from git; the rules for
+# distcheck will take care of turning this on when making a release
+m4_define([maintainer_flags_default], [m4_if(m4_eval(clutter_micro_version % 2), [1], [yes], [no])])
 AC_ARG_ENABLE([maintainer-flags],
-              [AC_HELP_STRING([--enable-maintainer-flags=@<:@no/yes@:>@],
+              [AC_HELP_STRING([--enable-maintainer-flags=@<:@no/yes/error@:>@],
                               [Use strict compiler flags @<:@default=maintainer_flags_default@:>@])],
               [],
               [enable_maintainer_flags=maintainer_flags_default])
 
-AS_IF([test "x$enable_maintainer_flags" = "xyes" && test "x$GCC" = "xyes"],
-      [
-        AS_COMPILER_FLAGS([MAINTAINER_CFLAGS],
-                          ["-Wall -Wshadow -Wcast-align -Wuninitialized
-                            -Wno-strict-aliasing -Wempty-body -Wformat
-                            -Wformat-security -Winit-self
-                            -Wdeclaration-after-statement -Wvla"])
-      ]
+MAINTAINER_COMPILER_FLAGS="-Wall -Wshadow -Wcast-align -Wuninitialized
+                           -Wno-strict-aliasing -Wempty-body -Wformat
+                           -Wformat-security -Winit-self
+                           -Wdeclaration-after-statement -Wvla"
+
+AS_CASE([$enable_maintainer_flags],
+        [yes],
+        [
+          AS_COMPILER_FLAGS([MAINTAINER_CFLAGS], [$MAINTAINER_COMPILER_FLAGS])
+        ],
+
+        [no],
+        [
+        ],
+
+        [error],
+        [
+          MAINTAINER_COMPILER_FLAGS="$MAINTAINER_COMPILER_FLAGS -Werror"
+          AS_COMPILER_FLAGS([MAINTAINER_CFLAGS], [$MAINTAINER_COMPILER_FLAGS])
+        ],
+
+        [*],
+        [AC_MSG_ERROR([Invalid option for --enable-maintainer-flags])]
 )
 
 AC_SUBST(MAINTAINER_CFLAGS)