libglade: Fix test for Linuxbrew
libglade: Fix test for Linuxbrew

libintl is only available on macOS.
Also, specify the correct backend for GDK and GTK+ depending on platform

Closes Linuxbrew/homebrew-core#931.

Signed-off-by: Bob W. Hogg <c772a964fd55352a3510e5d535dd9ccc9ac30168@linux.com>

rwhogg/homebrew-core,Linuxbrew/homebrew-core,maxim-belkin/homebrew-core,LinuxbrewTestBot/homebrew-core,LinuxbrewTestBot/homebrew-core,rwhogg/homebrew-core,maxim-belkin/homebrew-core,Linuxbrew/homebrew-core