Enable THREADS_PREFER_PTHREAD_FLAG only on UNIX

[wantehchang]

Call find_package(Threads) on all platforms.

Fixes #3262.

[wantehchang]

@kmilos @vtorri Please review.

[vtorri]

problem still here. With git took 2 minutes ago:

diff --git a/CMakeLists.txt b/CMakeLists.txt
index c9a7f82d..3db794f9 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -432,13 +432,14 @@ if(NOT WIN32 AND NOT APPLE)
     target_link_libraries(avif_obj PRIVATE m)
 endif()
 
-if(UNIX OR MINGW)
+if(UNIX)
     # Find out if we have threading available
     set(THREADS_PREFER_PTHREAD_FLAG ON)
-    find_package(Threads)
-    target_link_libraries(avif_obj PRIVATE Threads::Threads)
 endif()
 
+find_package(Threads)
+target_link_libraries(avif_obj PRIVATE Threads::Threads)
+
 if(NOT AVIF_LIBYUV_ENABLED)
     target_sources(
         avif_obj

compilation:

[2/26] Building C object CMakeFiles/avif_obj.dir/src/properties.c.obj
[3/26] Building C object CMakeFiles/avif_obj.dir/src/mem.c.obj
[4/26] Building C object CMakeFiles/avif_obj.dir/src/reformat_libsharpyuv.c.obj
[5/26] Building C object CMakeFiles/avif_obj.dir/src/exif.c.obj
[6/26] Building C object CMakeFiles/avif_obj.dir/src/rawdata.c.obj
[7/26] Building C object CMakeFiles/avif_obj.dir/src/io.c.obj
[8/26] Building C object CMakeFiles/avif_obj.dir/src/colrconvert.c.obj
[9/26] Building C object CMakeFiles/avif_obj.dir/src/obu.c.obj
[10/26] Building C object CMakeFiles/avif_obj.dir/src/scale.c.obj
[11/26] Building C object CMakeFiles/avif_obj.dir/src/sampletransform.c.obj
[12/26] Building C object CMakeFiles/avif_obj.dir/src/alpha.c.obj
[13/26] Building C object CMakeFiles/avif_obj.dir/src/codec_dav1d.c.obj
[14/26] Building C object CMakeFiles/avif_obj.dir/src/utils.c.obj
[15/26] Building C object CMakeFiles/avif_obj.dir/src/colr.c.obj
[16/26] Building C object CMakeFiles/avif_obj.dir/src/reformat_libyuv.c.obj
[17/26] Building C object CMakeFiles/avif_obj.dir/src/gainmap.c.obj
[18/26] Building C object CMakeFiles/avif_obj.dir/src/stream.c.obj
[19/26] Building C object CMakeFiles/avif_obj.dir/src/codec_aom.c.obj
[20/26] Building C object CMakeFiles/avif_obj.dir/src/avif.c.obj
[21/26] Building C object CMakeFiles/avif_obj.dir/src/write.c.obj
[22/26] Building C object CMakeFiles/avif_obj.dir/src/reformat.c.obj
[23/26] Building C object CMakeFiles/avif_obj.dir/src/read.c.obj
[24/26] Linking C static library libavif_internal.a
[25/26] Linking CXX shared library libavif.dll
FAILED: libavif.dll libavif.dll.a 
: && /usr/bin/x86_64-w64-mingw32-g++ -I/home/vtorri/ewpi_64/include -D__USE_MINGW_ANSI_STDIO=0 -D_WIN32_WINNT=0x0A00 -O2 -pipe -march=native  -L/home/vtorri/ewpi_64/lib -shared -o libavif.dll -Wl,--out-implib,libavif.dll.a -Wl,--major-image-version,16,--minor-image-version,4 CMakeFiles/avif_obj.dir/src/alpha.c.obj CMakeFiles/avif_obj.dir/src/avif.c.obj CMakeFiles/avif_obj.dir/src/colr.c.obj CMakeFiles/avif_obj.dir/src/colrconvert.c.obj CMakeFiles/avif_obj.dir/src/diag.c.obj CMakeFiles/avif_obj.dir/src/exif.c.obj CMakeFiles/avif_obj.dir/src/gainmap.c.obj CMakeFiles/avif_obj.dir/src/io.c.obj CMakeFiles/avif_obj.dir/src/mem.c.obj CMakeFiles/avif_obj.dir/src/obu.c.obj CMakeFiles/avif_obj.dir/src/properties.c.obj CMakeFiles/avif_obj.dir/src/rawdata.c.obj CMakeFiles/avif_obj.dir/src/read.c.obj CMakeFiles/avif_obj.dir/src/reformat.c.obj CMakeFiles/avif_obj.dir/src/reformat_libsharpyuv.c.obj CMakeFiles/avif_obj.dir/src/reformat_libyuv.c.obj CMakeFiles/avif_obj.dir/src/sampletransform.c.obj CMakeFiles/avif_obj.dir/src/scale.c.obj CMakeFiles/avif_obj.dir/src/stream.c.obj CMakeFiles/avif_obj.dir/src/utils.c.obj CMakeFiles/avif_obj.dir/src/write.c.obj CMakeFiles/avif_obj.dir/src/codec_dav1d.c.obj CMakeFiles/avif_obj.dir/src/codec_aom.c.obj  /home/vtorri/ewpi_64/lib/libyuv.dll.a  -lpthread  /home/vtorri/ewpi_64/lib/libdav1d.dll.a  /home/vtorri/ewpi_64/lib/libaom.dll.a  -l_aom_dep_lib_m-NOTFOUND  -l_aom_dep_lib_pthread-NOTFOUND  /home/vtorri/ewpi_64/lib/libvmaf.dll.a  -l_aom_dep_lib_m-NOTFOUND  -l_aom_dep_lib_pthread-NOTFOUND  /home/vtorri/ewpi_64/lib/libvmaf.dll.a  -lkernel32 -luser32 -lgdi32 -lwinspool -lshell32 -lole32 -loleaut32 -luuid -lcomdlg32 -ladvapi32 && :
/usr/bin/x86_64-w64-mingw32-ld : ne peut pas trouver -l_aom_dep_lib_m-NOTFOUND : Aucun fichier ou dossier de ce nom
/usr/bin/x86_64-w64-mingw32-ld : ne peut pas trouver -l_aom_dep_lib_pthread-NOTFOUND : Aucun fichier ou dossier de ce nom
/usr/bin/x86_64-w64-mingw32-ld : ne peut pas trouver -l_aom_dep_lib_m-NOTFOUND : Aucun fichier ou dossier de ce nom
/usr/bin/x86_64-w64-mingw32-ld : ne peut pas trouver -l_aom_dep_lib_pthread-NOTFOUND : Aucun fichier ou dossier de ce nom
collect2: error: ld returned 1 exit status
ninja: build stopped: subcommand failed.

[kmilos]

This is completely unrelated to the aom overlinking problem, which needs to be addressed there.