fltk: Depends on x11 and mesa for Linuxbrew
fltk: Depends on x11 and mesa for Linuxbrew

Fix error: Configure could not find required X11 libraries, aborting.

Closes Linuxbrew/homebrew-core#1777.

Signed-off-by: Shaun Jackman <b580dab3251a9622aba3803114310c23fdb42900@gmail.com>

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