meson: don't add -pthread to static linking flags on Windows

Meson always returns -pthread in dependency('threads') on non-MSVC
compilers.  On Windows we use Windows threading primitives, so we don't
need this.  Avoid adding -pthread to libzstd's link flags, either as a
Meson subproject or via pkg-config Libs.private, so the application
doesn't inadvertently depend on winpthreads.

Add a Meson MinGW cross-compile CI test that checks for this.  It turns
out that pzstd fails to build in that environment, so have the test
skip building contrib for now.

build/meson/meson.build

@@ -88,8 +88,13 @@ feature_lz4 = get_option('lz4')
 # =============================================================================
 
 libm_dep = cc.find_library('m', required: false)
-thread_dep = dependency('threads', required: feature_multi_thread)
-use_multi_thread = thread_dep.found()
+if host_machine_os == os_windows
+  thread_dep = dependency('', required: false)
+  use_multi_thread = not feature_multi_thread.disabled()
+else
+  thread_dep = dependency('threads', required: feature_multi_thread)
+  use_multi_thread = thread_dep.found()
+endif
 # Arguments in dependency should be equivalent to those passed to pkg-config
 zlib_dep = dependency('zlib', required: feature_zlib)
 use_zlib = zlib_dep.found()

build/meson/meson_options.txt

@@ -27,7 +27,7 @@ option('bin_contrib', type: 'boolean', value: false,
   description: 'Enable contrib build')
 
 option('multi_thread', type: 'feature', value: 'enabled',
-  description: 'Enable multi-threading when pthread is detected')
+  description: 'Enable multi-threading when pthread or Windows is detected')
 option('zlib', type: 'feature', value: 'auto',
   description: 'Enable zlib support')
 option('lzma', type: 'feature', value: 'auto',