configure: Check for Fox 1.7 as well as Fox 1.6 for the testgui

The testgui will work with both, so check for them both at
configure time.

This came from Alex Barker <alex@1stleg.com> and was posted on github.

configure.ac

@@ -196,7 +196,7 @@ if test "x$testgui_enabled" != "xno"; then
 		fi
 	else
 		# On Linux and FreeBSD platforms, use pkg-config to find fox.
-		PKG_CHECK_MODULES([fox], [fox])
+		PKG_CHECK_MODULES([fox], [fox17], [], [PKG_CHECK_MODULES([fox], [fox])])
 		LIBS_TESTGUI="${LIBS_TESTGUI} $fox_LIBS"
 		if test "x$os" = xfreebsd; then
 			LIBS_TESTGUI="${LIBS_TESTGUI} -L/usr/local/lib"