gallium: Fix automake for Android (v2)

gallium: Fix automake for Android (v2)

Chromium OS uses Autotools and pkg-config when building Mesa for
Android. The gallium drivers were failing to find the headers and
libraries for zlib and Android's libbacktrace.

v2:
  - Don't add a check for zlib.pc. configure.ac already checks for
    zlib.pc elsewhere. [for tfiga]
  - Check for backtrace.pc separately from the other Android libs.
    [for tfiga]

Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>