CMake: prefer -pthread over -lpthread.

CMake ewhy can't you have this as a default?

src/Magnum/GL/Test/CMakeLists.txt

@@ -125,6 +125,7 @@ if(BUILD_GL_TESTS)
 
     corrade_add_test(GLContextGLTest ContextGLTest.cpp LIBRARIES MagnumOpenGLTester)
     if(NOT CORRADE_TARGET_EMSCRIPTEN)
+        set(THREADS_PREFER_PTHREAD_FLAG TRUE)
         find_package(Threads REQUIRED)
         target_link_libraries(GLContextGLTest PRIVATE Threads::Threads)
     endif()

src/Magnum/Trade/CMakeLists.txt

@@ -150,6 +150,7 @@ install(TARGETS MagnumTrade
 install(FILES ${MagnumTrade_HEADERS} DESTINATION ${MAGNUM_INCLUDE_INSTALL_DIR}/Trade)
 
 if(WITH_IMAGECONVERTER)
+    set(THREADS_PREFER_PTHREAD_FLAG TRUE)
     find_package(Threads REQUIRED)
 
     add_executable(magnum-imageconverter imageconverter.cpp)

src/MagnumPlugins/AnyImageConverter/Test/CMakeLists.txt

@@ -25,6 +25,7 @@
 
 # Needed to test verbose output from OpenExrImageConverter (detected thread
 # count), unfortunately no better option for testing flags right now
+set(THREADS_PREFER_PTHREAD_FLAG TRUE)
 find_package(Threads REQUIRED)
 
 if(CORRADE_TARGET_EMSCRIPTEN OR CORRADE_TARGET_ANDROID)