CDRIVER-4776 add thread library to pkg-config Libs (#1477)

* remove unused `gx_linkopts`

* add thread library to pkg-config libs

Co-authored-by: vector-of-bool <vectorofbool@gmail.com>

CMakeLists.txt

@@ -228,6 +228,8 @@ set(THREADS_PREFER_PTHREAD_FLAG TRUE)
 find_package(Threads REQUIRED)
 mongo_platform_use_target(Threads::Threads)
 
+set_property(DIRECTORY APPEND PROPERTY pkg_config_LIBS ${CMAKE_THREAD_LIBS_INIT})
+
 if(WIN32)
    # Required for gethostbyname on Windows:
    mongo_platform_link_libraries(ws2_32)

build/cmake/GeneratePkgConfig.cmake

@@ -286,7 +286,7 @@ function(_generate_pkg_config_content out)
     # XXX: Could we define a genex that can transform the INTERFACE_LINK_LIBRARIES to a list of
     #      pkg-config-compatible "-l"-flags? That would remove the need to populate pkg_config_LIBS
     #      manually, and instead rely on target properties to handle transitive dependencies.
-    string(APPEND libs "$<JOIN:${gx_libs};${gx_linkopts}, >")
+    string(APPEND libs "$<JOIN:${gx_libs}, >")
 
     # Cflags:
     set(cflags)