meson: Don't probe for pthread on windows

Always use the compatibility layer.
Fix #28

meson.build

@@ -32,10 +32,11 @@ cdata = configuration_data()
 cdata_asm = configuration_data()
 cc = meson.get_compiler('c')
 
-if not meson.is_cross_build() 
+# On windows, we use a compatibility layer to emulate pthread
+if host_machine.system() != 'windows'
     thread_dependency = dependency('threads')
 else
-    thread_dependency = cc.find_library('pthread', required: false)
+    thread_dependency = declare_dependency(sources: ['src/win32/thread.c'])
 endif
 
 dav1d_inc_dirs = include_directories(['include', 'include/dav1d'])
@@ -264,10 +265,6 @@ else
     nasm_objs = []
 endif
 
-if host_machine.system() == 'windows'
-    libdav1d_sources += files('src/win32/thread.c')
-endif
-
 # Build a helper library for each bitdepth
 bitdepth_objs = []
 foreach bitdepth : dav1d_bitdepths