(svn r25054) -Document: the *FLAGS_BUILD in configure

config.lib

@@ -3648,6 +3648,9 @@ showhelp() {
 	echo "  LDFLAGS                        linker flags, e.g. -L<lib dir> if you"
 	echo "                                 have libraries in a nonstandard"
 	echo "                                 directory <lib dir>"
+	echo "  CFLAGS_BUILD                   C compiler flags for build time tool generation"
+	echo "  CXXFLAGS_BUILD                 C++ compiler flags for build time tool generation"
+	echo "  LDFLAGS_BUILD                  linker flags for build time tool generation"
 	echo ""
 	echo "Use these variables to override the choices made by 'configure' or to help"
 	echo "it to find libraries and programs with nonstandard names/locations."