cmake: Silence warnings.

[orbea]

Silences many warnings with newer cmake versions, tested with cmake-3.18.2.

There are two issues.

• There are many instances of find_package(Foo) which calls FindFoo.cmake and includes many FOO_ variables. It should use one name consistently. For example find_package(FOO), FindFOO.cmake and FOO_FOUND.
• It seems better to use find_package(PkgConfig) instead of include(FindPkgConfig), most of the cmake files in dolphin already do this and upstream cmake also does this in their own modules.

Examples:

CMake Warning (dev) at /usr/share/cmake-3.18/Modules/FindPackageHandleStandardArgs.cmake:273 (message):
  The package name passed to `find_package_handle_standard_args` (CUBEB) does
  not match the name of the calling package (Cubeb).  This can lead to
  problems in calling code that expects `find_package` result variables
  (e.g., `_FOUND`) to follow a certain pattern.
Call Stack (most recent call first):
  CMake/FindCubeb.cmake:6 (find_package_handle_standard_args)
  CMakeLists.txt:682 (find_package)
This warning is for project developers.  Use -Wno-dev to suppress it.

CMake Warning (dev) at /usr/share/cmake-3.18/Modules/FindPackageHandleStandardArgs.cmake:273 (message):
  The package name passed to `find_package_handle_standard_args` (PkgConfig)
  does not match the name of the calling package (PulseAudio).  This can lead
  to problems in calling code that expects `find_package` result variables
  (e.g., `_FOUND`) to follow a certain pattern.
Call Stack (most recent call first):
  /usr/share/cmake-3.18/Modules/FindPkgConfig.cmake:59 (find_package_handle_standard_args)
  CMake/FindPulseAudio.cmake:9 (include)
  Source/Core/AudioCommon/CMakeLists.txt:51 (find_package)
This warning is for project developers.  Use -Wno-dev to suppress it.

Full log: dolphin.log

[iwubcode]

Not really for this review but just surprised we use if(LIBUDEV_FOUND...) when we specify REQURIED

[orbea]

Like you said, that is out of scope for this PR, but it should probably be fixed later... It might be possible to make them entirely optional?

$ find . -type f -iname '*cmake*' | xargs grep -e LIBUDEV_FOUND -e LIBEVDEV_FOUND
./Externals/libusb/CMakeLists.txt:    if(LIBUDEV_FOUND)
./Externals/hidapi/CMakeLists.txt:  if(CMAKE_SYSTEM_NAME STREQUAL "Linux" AND LIBUDEV_FOUND)
./Source/Core/InputCommon/CMakeLists.txt:if(LIBEVDEV_FOUND AND LIBUDEV_FOUND)
./CMake/FindLIBEVDEV.cmake:#  LIBEVDEV_FOUND - System has libevdev
./CMake/FindLIBUDEV.cmake:#  LIBUDEV_FOUND - System has LIBUDEV
./CMakeLists.txt:  if(LIBUDEV_FOUND AND LIBEVDEV_FOUND)