* CMakeLists.txt: Include 'FindPkgConfig' module.

Older cmake versions don't provide `pkg_check_modules` by default.

Fixes #1126.

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 4590309..8cb7783 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -122,6 +122,7 @@
 
 include(CheckIncludeFile)
 include(CMakeDependentOption)
+include(FindPkgConfig)
 
 # CMAKE_TOOLCHAIN_FILE must be set before `project' is called, which
 # configures the base build environment and references the toolchain file