libxcb: correct dependency types
libxcb: correct dependency types

1. libpthread-stubs is required by libxcb (in xcb.pc)
2. libxdmcp was a recommended dependency, but no configure-time switches
depended on it. Hence, we remove it (for now).

Signed-off-by: Maxim Belkin <032baa6ccf1d4eaa4536fc4f9b83b9a1f593449a@gmail.com>

LinuxbrewTestBot/homebrew-xorg,Linuxbrew/homebrew-xorg,maxim-belkin/homebrew-xorg