build: Some warnings should always stop compilation

Not every single one, but things that we care about to avoid breaking
build on other platforms, or for security issues, should not be allowed.

We should not force everyone to use -Werror, though; and we should still
allow building Clutter without any special flag.

configure.ac

@@ -920,13 +920,20 @@ AC_ARG_ENABLE([maintainer-flags],
               [enable_maintainer_flags=maintainer_flags_default])
 
 MAINTAINER_COMPILER_FLAGS="$MAINTAINER_COMPILER_FLAGS
-                           -Wall -Wcast-align -Wuninitialized
-                           -Wno-strict-aliasing -Wempty-body -Wformat
-                           -Wformat-security -Wformat-nonliteral -Winit-self
-                           -Wdeclaration-after-statement -Wvla
-                           -Wpointer-arith -Wmissing-declarations
+                           -Wall
                            -Wcast-align
-                           -Wredundant-decls"
+                           -Wuninitialized
+                           -Wno-strict-aliasing
+                           -Werror=pointer-arith
+                           -Werror=missing-declarations
+                           -Werror=redundant-decls
+                           -Werror=empty-body
+                           -Werror=format
+                           -Werror=format-security
+                           -Werror=format-nonliteral
+                           -Werror=init-self
+                           -Werror=declaration-after-statement
+                           -Werror=vla"
 
 AS_CASE([$enable_maintainer_flags],
         [yes],