build-sys: really override CFLAGS for gtk-doc

In 29a00c41 an override was added, but commandline variables have
higher precedence than Makefile variables, so the override was not
effective for commandline variables.

While at it, duplicate for libudev.

docs/gudev/Makefile.am

@@ -77,8 +77,8 @@ expand_content_files=
 # be listed only once.
 # Kill CFLAGS here because gtk-doc thinks adding CFLAGS to CC _and_ also
 # adding CFLAGS itself again would work.
-CFLAGS=
-LDFLAGS=
+override CFLAGS=
+override LDFLAGS=
 
 # CFLAGS and LDFLAGS for compiling gtkdoc-scangobj with your library.
 # Only needed if you are using gtkdoc-scangobj to dynamically query widget

docs/libudev/Makefile.am

@@ -72,6 +72,14 @@ content_files = version.xml
 # e.g. expand_content_files=running.sgml
 expand_content_files=
 
+# Hack, hack. You silly gtk-doc, you must not add  CFLAGS multiple
+# times when calling gcc; it surely can not work with options that must
+# be listed only once.
+# Kill CFLAGS here because gtk-doc thinks adding CFLAGS to CC _and_ also
+# adding CFLAGS itself again would work.
+override CFLAGS=
+override LDFLAGS=
+
 # CFLAGS and LDFLAGS for compiling gtkdoc-scangobj with your library.
 # Only needed if you are using gtkdoc-scangobj to dynamically query widget
 # signals and properties.