cmake: Include "-pthread" in the pkg-config file if using pthreads (#832)

This makes sure that users of the library pass the right linker
flags if linking libflac statically, with a mingw toolchain that
doesn't automatically link in pthreads.

Author: mstorsjo

CMakeLists.txt

@@ -173,7 +173,7 @@ if(MINGW AND (WITH_FORTIFY_SOURCE OR WITH_STACK_PROTECTOR))
     link_libraries("ssp.a")
     # static libraries don't carry over other static libraries in mingw
     # we need to export it in the pkg-config
-    set(FLAC_STATIC_LIBS "-lssp")
+    set(FLAC_STATIC_LIBS "${FLAC_STATIC_LIBS} -lssp")
   endif()
 elseif(NOT MSVC)
   set(HAVE_LIBSSP 1)
@@ -228,6 +228,7 @@ if(ENABLE_MULTITHREADING)
     find_package(Threads)
     if(CMAKE_USE_PTHREADS_INIT)
         set(HAVE_PTHREAD 1)
+        set(FLAC_STATIC_LIBS "${FLAC_STATIC_LIBS} -pthread")
     else()
         if(HAVE_THREADS_H)
             set(HAVE_C11THREADS 1)