meson.build: Fix building with cross SDK

meson.build: Fix building with cross SDK

For some time now, yocto/OE no longer builds all weston back-ends, which
means the X11 is no longer supported on AGL platform.

The agl-compositor can be built, assuming all dependencies have been
satisfied, without the need of having an AGL or yocto/OE environment,
but so far, I've never tried using SDK directly.

This patch should allow building the compositor, by only using the AGL
SDK, and instead of using the local pkg-config, only use what is
available in the SDK sysroot. This takes care of assuming that X11
back-end is available, when in reality, in the AGL SDK platform that is
not the case.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: Ia2a182b9c3015f14f82043a04e83e917cd99bf6f