Fix some tests.

- add cmp and convert utilities.
- use m with gcc and pthreads with clang

.github/workflows/ci-mingw.yml

@@ -38,8 +38,10 @@ jobs:
         pacboy: >-
           aom:p
           cmake:p
+          diffutils:p
           cc:p
           dav1d:p
+          imagemagick:p
           libjpeg-turbo:p
           libpng:p
           libyuv:p

CMakeLists.txt

@@ -431,14 +431,11 @@ endif()
 # /usr/local/include.
 set(CMAKE_FIND_FRAMEWORK LAST)
 
-if(UNIX OR (MINGW AND CMAKE_C_COMPILER_ID MATCHES "Clang"))
+if(UNIX OR MINGW)
     # Find out if we have threading available
     set(THREADS_PREFER_PTHREAD_FLAG ON)
     find_package(Threads)
-    set(AVIF_PLATFORM_LIBRARIES ${AVIF_PLATFORM_LIBRARIES} Threads::Threads)
-endif()
-if(UNIX)
-    set(AVIF_PLATFORM_LIBRARIES ${AVIF_PLATFORM_LIBRARIES} m)
+    set(AVIF_PLATFORM_LIBRARIES ${AVIF_PLATFORM_LIBRARIES} m Threads::Threads)
 endif()
 
 set(AVIF_CODEC_DEFINITIONS)