Make dependency paths relocatable

The generated configurations for both CMake and pkg-config included
absolute paths to dependencies (OpenSSL, MbedTLS). This is contrary
to the general CMake advise to create relocatable packages [1].

Additionally, when building both mbedtls and libevent via CMake's
FetchContent in the same project, loading the project would fail with

  INTERFACE_INCLUDE_DIRECTORIES property contains path:
    "/home/user/project/cmake-build/_deps/mbedtls-build/include"
  which is prefixed in the source directory.

The required changes include:
- Adding the outer includes only to the BUILD_INTERFACE solves the
  makes the CMake paths relocatable and thus solves the FetchContent
  problem.
- Updates to libevent_*.pc.in fixes the relocatable issues for
  pkg-config and properly declares currently missing dependencies.
- Using components for linking to OpenSSL (requiring CMake 3.4)
  and MbedTLS. The new MbedTLS target names now match the component
  names of the MbedTLS' CMake project.
- Use the Threads CMake library reference instead of a direct
  reference to support both built-in pthread and -lpthread.

v2 (azat): get back CMAKE_REQUIRED_LIBRARIES

[1] https://cmake.org/cmake/help/v3.25/manual/cmake-packages.7.html#creating-relocatable-packages

2 files changed, 18 insertions, 3 deletions

diff --git a/test-export/CMakeLists.txt b/test-export/CMakeLists.txt
index 89c1806c..84e15181 100644
--- a/test-export/CMakeLists.txt
+++ b/test-export/CMakeLists.txt
@@ -1,7 +1,8 @@
-cmake_minimum_required(VERSION 3.1.2)
+cmake_minimum_required(VERSION 3.4)
 if (POLICY CMP0074)
     cmake_policy(SET CMP0074 NEW)
 endif()
+list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_LIST_DIR}/../cmake")
 project(verify)
 # set(CMAKE_VERBOSE_MAKEFILE 1)
 if(NOT ${EVENT__CODE_COMPONENT} STREQUAL "")
diff --git a/test-export/test-export.py b/test-export/test-export.py
index 84824350..efeac0cc 100644
--- a/test-export/test-export.py
+++ b/test-export/test-export.py
@@ -46,10 +46,24 @@ def link_and_run(link, code):
     """
     exec_cmd("cmake --build . -v --target clean", True)
     arch = ''
+    vcpkg = ''
+    openssldir = ''
+    mbedtlsdir = ''
     if platform.system() == "Windows":
         arch = '-A x64'
-    cmd = 'cmake .. %s -DEVENT__LINK_COMPONENT=%s -DEVENT__CODE_COMPONENT=%s' % (
-        arch, link, code)
+        vcpkg_root = os.environ.get('VCPKG_ROOT')
+        if vcpkg_root is not None:
+            vcpkg = f"-DCMAKE_TOOLCHAIN_FILE={vcpkg_root}/scripts/buildsystems/vcpkg.cmake"
+    elif platform.system() == "Darwin":
+        openssldir = '-DOPENSSL_ROOT_DIR=/usr/local/opt/openssl'
+        mbedtlsdir = '-DMBEDTLS_ROOT_DIR=/usr/local/opt/mbedtls@2'
+    cmd = f"cmake .." \
+          f" {arch}" \
+          f" {vcpkg}" \
+          f" -DEVENT__LINK_COMPONENT={link}" \
+          f" -DEVENT__CODE_COMPONENT={code}" \
+          f" {openssldir}" \
+          f" {mbedtlsdir}"
     if link_type == "static":
         cmd = "".join([cmd, " -DLIBEVENT_STATIC_LINK=1"])
     r = exec_cmd(cmd, True)