cmake: Fix exporting -pthread to static pkgconfig (#965)

Before this change, libcares.pc contained ` -l-pthread`.  

Signed-off-by: Kai Pastor (@dg0yt)

CMakeLists.txt

@@ -509,11 +509,7 @@ IF (CARES_THREADS)
 			CARES_EXTRAINCLUDE_IFSET (HAVE_PTHREAD_H                       pthread.h)
 			CARES_EXTRAINCLUDE_IFSET (HAVE_PTHREAD_NP_H                    pthread_np.h)
 			CHECK_SYMBOL_EXISTS (pthread_init  "${CMAKE_EXTRA_INCLUDE_FILES}" HAVE_PTHREAD_INIT)
-			# Make sure libcares.pc.cmake knows about thread libraries on static builds
-			# The variable set by FIND_PACKAGE(Threads) has a -l prefix on it, we need
-			# to strip that first since CARES_DEPENDENT_LIBS doesn't expect that.
-			STRING (REPLACE "-l" "" CARES_THREAD_LIBRARY "${CMAKE_THREAD_LIBS_INIT}")
-			LIST (APPEND CARES_DEPENDENT_LIBS ${CARES_THREAD_LIBRARY})
+			LIST (APPEND CARES_DEPENDENT_LIBS ${CMAKE_THREAD_LIBS_INIT})
 		ELSE ()
 			MESSAGE (WARNING "Threading support not found, disabling...")
 			SET (CARES_THREADS OFF)
@@ -783,7 +779,10 @@ IF (CARES_INSTALL)
 
 	# pkgconfig support for static builds
 	FOREACH (LIB ${CARES_DEPENDENT_LIBS})
-		SET (CARES_PRIVATE_LIBS "${CARES_PRIVATE_LIBS} -l${LIB}")
+		IF (NOT LIB MATCHES "^-")
+			SET (LIB "-l${LIB}")
+		ENDIF ()
+		SET (CARES_PRIVATE_LIBS "${CARES_PRIVATE_LIBS} ${LIB}")
 	ENDFOREACH ()
 
 	CONFIGURE_FILE("libcares.pc.cmake" "libcares.pc" @ONLY)