meson: fix MSVC support

Regression from commit a5f2c45528. It is
not possible to unconditionally add the asm sources, since not all
compilers understand the .s file extension.

Specifically for meson, only compilers inheriting from the GNU mixin
will allow a .s file at configure time.

zstd doesn't support asm for MSVC for the same basic reason; if/when
Windows asm support is added, it would involve preprocessing with nasm,
most likely.

build/meson/lib/meson.build

@@ -37,7 +37,6 @@ libzstd_sources = [join_paths(zstd_rootdir, 'lib/common/entropy_common.c'),
   join_paths(zstd_rootdir, 'lib/compress/zstd_opt.c'),
   join_paths(zstd_rootdir, 'lib/compress/zstd_ldm.c'),
   join_paths(zstd_rootdir, 'lib/decompress/huf_decompress.c'),
-  join_paths(zstd_rootdir, 'lib/decompress/huf_decompress_amd64.S'),
   join_paths(zstd_rootdir, 'lib/decompress/zstd_decompress.c'),
   join_paths(zstd_rootdir, 'lib/decompress/zstd_decompress_block.c'),
   join_paths(zstd_rootdir, 'lib/decompress/zstd_ddict.c'),
@@ -46,6 +45,12 @@ libzstd_sources = [join_paths(zstd_rootdir, 'lib/common/entropy_common.c'),
   join_paths(zstd_rootdir, 'lib/dictBuilder/divsufsort.c'),
   join_paths(zstd_rootdir, 'lib/dictBuilder/zdict.c')]
 
+# really we need anything that defines __GNUC__ as that is what ZSTD_ASM_SUPPORTED is gated on
+# but these are the two compilers that are supported in tree and actually handle this correctly
+if [compiler_gcc, compiler_clang].contains(cc_id)
+  libzstd_sources += join_paths(zstd_rootdir, 'lib/decompress/huf_decompress_amd64.S')
+endif
+
 # Explicit define legacy support
 add_project_arguments('-DZSTD_LEGACY_SUPPORT=@0@'.format(legacy_level),
   language: 'c')