egl: Use pkg-config for Android NDK build

egl: Use pkg-config for Android NDK build

It's possible to build Mesa for Android using the traditional
autotools workflow [1]. ChromiumOS fetches Android prebuilts and
puts them in a sysroot. We now want to use pkg-config to specify
the location of system headers and libraries [2].

To enable this, let's add the required pkg-config checks and link
against them.

[1] https://developer.android.com/ndk/guides/standalone_toolchain.html
[2] https://chromium-review.googlesource.com/#/c/403237/

v2: Bundle pkg-config checks together (Emil)
v3: Provide further context on standalone NDK Mesa build (Emil)
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>