Merge #929 -- cmake package improvements

Example:

  find_package(Libevent 2.2.0 REQUIRED COMPONENTS core)
  add_executable(test test.c)
  target_link_libraries(test ${LIBEVENT_LIBRARIES})

* upstream/pr/929:
  CI: integrate testing for components export
  cmake: test for find_package()
  cmake: improve package config file

diff --cc appveyor.yml
index ec9e36b518894013e08d3bd6cce3c0cd557b7f92,a2687237e20407e16f2e3375bc74944250ffacd9..24c0b0c974ffb37a32e36209b41fb14d8dd745b2
--- 1/appveyor.yml
--- 2/appveyor.yml
+++ b/appveyor.yml
@@@ -23,9 -23,9 +23,10 @@@ environment
    global:
      APPVEYOR_SAVE_CACHE_ON_ERROR: true
      OPENSSL_ROOT: C:/OpenSSL-Win64
+     PYTHON3: C:/Python37-x64/python.exe
      MPATH: C:/mingw-w64/x86_64-7.2.0-posix-seh-rt_v5-rev1/mingw64/bin;C:/msys64/usr/bin
 -    EVENT_TESTS_PARALLEL: 20
 +    # Do not run tests in parallel to reduce false-positive
 +    EVENT_TESTS_PARALLEL: 1
      EVENT_BUILD_PARALLEL: 10
    matrix:
      - EVENT_BUILD_METHOD: "cmake"