Add benchmark_main.pc to link main() containing library (#1779)

This is similar to the addition in google/googletest@8604c4a#diff-eb8e49bdf5e9aafb996777a4f4302ad1efd281222bf3202eb9b77ce47496c345
that added pkg-config support in GTest. Without this, users
need to manually find the library containing `main()`.

cmake/benchmark_main.pc.in

@@ -0,0 +1,7 @@
+libdir=@CMAKE_INSTALL_FULL_LIBDIR@
+
+Name: @PROJECT_NAME@
+Description: Google microbenchmark framework (with main() function)
+Version: @VERSION@
+Requires: benchmark
+Libs: -L${libdir} -lbenchmark_main

src/CMakeLists.txt

@@ -86,6 +86,7 @@ set(generated_dir "${PROJECT_BINARY_DIR}")
 set(version_config "${generated_dir}/${PROJECT_NAME}ConfigVersion.cmake")
 set(project_config "${generated_dir}/${PROJECT_NAME}Config.cmake")
 set(pkg_config "${generated_dir}/${PROJECT_NAME}.pc")
+set(pkg_config_main "${generated_dir}/${PROJECT_NAME}_main.pc")
 set(targets_to_export benchmark benchmark_main)
 set(targets_export_name "${PROJECT_NAME}Targets")
 
@@ -105,6 +106,7 @@ write_basic_package_version_file(
 )
 
 configure_file("${PROJECT_SOURCE_DIR}/cmake/benchmark.pc.in" "${pkg_config}" @ONLY)
+configure_file("${PROJECT_SOURCE_DIR}/cmake/benchmark_main.pc.in" "${pkg_config_main}" @ONLY)
 
 export (
   TARGETS ${targets_to_export}
@@ -133,7 +135,7 @@ if (BENCHMARK_ENABLE_INSTALL)
       DESTINATION "${CMAKE_INSTALL_LIBDIR}/cmake/${PROJECT_NAME}")
 
   install(
-      FILES "${pkg_config}"
+      FILES "${pkg_config}" "${pkg_config_main}"
       DESTINATION "${CMAKE_INSTALL_LIBDIR}/pkgconfig")
 
   install(