CI: Test consuming the exported CMake configs.

tests/cmake/CMakeLists.txt

@@ -0,0 +1,37 @@
+cmake_minimum_required(VERSION 3.5)
+
+project(LibreSSL_Consumer LANGUAGES C)
+
+find_package(
+  LibreSSL
+  CONFIG
+  REQUIRED
+  COMPONENTS Crypto SSL TLS
+)
+
+set(RESULTS_TO_CHECK
+  "LIBRESSL_VERSION"
+  "LIBRESSL_FOUND"
+  "LIBRESSL_INCLUDE_DIR"
+  "LIBRESSL_LIBRARIES"
+  "LIBRESSL_CRYPTO_LIBRARY"
+  "LIBRESSL_SSL_LIBRARY"
+  "LIBRESSL_TLS_LIBRARY"
+)
+
+foreach(RESULT_VAR IN LISTS RESULTS_TO_CHECK)
+  if(${RESULT_VAR})
+    message(STATUS "${RESULT_VAR}: ${${RESULT_VAR}}")
+  else()
+    message(FATAL_ERROR "${RESULT_VAR} was not set by the package.")
+  endif()
+endforeach()
+
+add_executable(crypto crypto.c)
+target_link_libraries(crypto PRIVATE LibreSSL::Crypto)
+
+add_executable(ssl ssl.c)
+target_link_libraries(ssl PRIVATE LibreSSL::SSL)
+
+add_executable(tls tls.c)
+target_link_libraries(tls PRIVATE LibreSSL::TLS)