Use FindPkgConfig to search for modules before doing manual search

[madebr]

• Look for libraries and include directories using FindPkgConfig first, before doing a manual search using find_library and find_path
• FindLibidn.cmake is not available, so remove all references and usages of LibIdn_xxx variables or Libidn::Libidn target

Fixes #487
Used in conan-io/conan-center-index#2903

[madebr]

Tests are failing because of #490

[wtoorop]

@banburybill can you review this? Thanks!

[wtoorop]

@madebr I took this for a spin, but cmake gave errors with me:

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 (Libidn2).  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/modules/FindLibidn2.cmake:31 (include)
  CMakeLists.txt:400 (find_package)
This warning is for project developers.  Use -Wno-dev to suppress it.

-- Found PkgConfig: /usr/bin/pkg-config (found version "0.29.2") 
-- Checking for module 'libidn2'
--   Found libidn2, version 2.3.0
CMake Error at /usr/share/cmake-3.18/Modules/FindPackageHandleStandardArgs.cmake:165 (message):
  Could NOT find Libidn2 (missing: LIBIDN2_INCLUDE_DIR) (found suitable
  version "2.3.0", minimum required is "2.0.0")
Call Stack (most recent call first):
  /usr/share/cmake-3.18/Modules/FindPackageHandleStandardArgs.cmake:458 (_FPHSA_FAILURE_MESSAGE)
  cmake/modules/FindLibidn2.cmake:76 (find_package_handle_standard_args)
  CMakeLists.txt:400 (find_package)


-- Configuring incomplete, errors occurred!
See also "/home/willem/repos/getdns/build/CMakeFiles/CMakeOutput.log".
See also "/home/willem/repos/getdns/build/CMakeFiles/CMakeError.log".

(tried on Ubuntu 21.04)

It would be really nice to have this though, so if you can fix this, that would be highly appreciated and welcomed!

[madebr]

The issue was that pkg_check_modules returns an empty PkgLibIdn2_INCLUDE_DIRS variable.
This causes find_package_handle_standard_args to fail because LIBIDN2_INCLUDE_DIR is a required variable.
Fix this by only calling find_package_handle_standard_args when not using pkg-config.

[wtoorop]

Thanks! Worked like a charm (at least for me ;))