gtk: Temporarily disable linking against cups
gtk: Temporarily disable linking against cups

Fixes error: Sorry, cups-config present but cups/cups.h missing.

Also, fix the test.

Closes Linuxbrew/homebrew-core#497.

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

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