[pkgconfig] cotire doesn't fully support cmake function FindPkgConfig

[JackBoosY]

When using pkg_check_modules, CMAKE_MINIMUM_REQUIRED_VERSION must be set greater than or equal to 3.1 to allow cmake to automatically add CMAKE_PREFIX_PATH to PKG_CONFIG_PATH so that pkg-config can be called to find .pc files.
However, the minimum cmake version requirement for cotire is 2.8.12, which will cause this setting to fail.

See code:
https://github.com/Kitware/CMake/blob/master/Modules/FindPkgConfig.cmake#L185-L188
https://github.com/Kitware/CMake/blob/master/Modules/FindPkgConfig.cmake#L383-L387

[JackBoosY]

It looks like cotire has not been updated for a long time, I opened this issue to remind other users that using pkgconfig while using cotire can cause this problem.
The workaround is to manually set PKG_CONFIG_USE_CMAKE_PREFIX_PATH to ON.