CMake: Use CMAKE_THREAD_LIBS_INIT in liblzma.pc only with pthreads

This shouldn't make much difference in practice as on Windows
no flags are needed anyway and unitialized variable (when threading
is disabled) expands to empty. But it's clearer this way.

(cherry picked from commit 2aecffe0f0e14f3ef635e8cd7b405420f2385de2)

CMakeLists.txt

@@ -1412,7 +1412,13 @@ else()
     set(includedir "${CMAKE_INSTALL_FULL_INCLUDEDIR}")
 endif()
 
-set(PTHREAD_CFLAGS "${CMAKE_THREAD_LIBS_INIT}")
+# Threads::Threads is linked in only when using POSIX threads.
+# Use an empty value if using Windows threads or if threading is disabled.
+set(PTHREAD_CFLAGS)
+if(USE_POSIX_THREADS)
+    set(PTHREAD_CFLAGS "${CMAKE_THREAD_LIBS_INIT}")
+endif()
+
 configure_file(src/liblzma/liblzma.pc.in liblzma.pc @ONLY)
 
 # Install the library binary. The INCLUDES specifies the include path that