Meson: add 'force_posix_threads' option

This allows building with posix threads on Windows. It is generally
better to use win32 threads implementation on Windows, but this option
can be used in case it causes issues, or for performance comparison for
example.

https://bugzilla.gnome.org/show_bug.cgi?id=784995

glib/meson.build

@@ -204,15 +204,15 @@ if host_system == 'windows'
   )
   glib_win_res = windows.compile_resources(glib_win_rc)
   glib_sources += [glib_win_res]
-  glib_sources += files('gthread-win32.c')
   glib_sources += files('gwin32.c', 'gspawn-win32.c', 'giowin32.c')
   platform_deps = [winsock2, cc.find_library('winmm')]
 else
-  glib_sources += files('gthread-posix.c')
   glib_sources += files('glib-unix.c', 'gspawn.c', 'giounix.c')
   platform_deps = []
 endif
 
+glib_sources += files('gthread-@0@.c'.format(threads_implementation))
+
 if enable_dtrace
   glib_dtrace_obj = dtrace_obj_gen.process('glib_probes.d')
   glib_dtrace_hdr = dtrace_hdr_gen.process('glib_probes.d')

meson.build

@@ -1476,15 +1476,16 @@ endif
 
 # === Threads ===
 
-# Let meson figure out all this business and whether -pthread or whatnot is needed
-# FIXME: probably needs more tweaking in meson for things like -D_REENTRANT etc.
-thread_dep = dependency('threads')
-
 # Determination of thread implementation
-if host_system == 'windows'
+if host_system == 'windows' and not get_option('force_posix_threads')
+  thread_dep = []
+  threads_implementation = 'win32'
   glibconfig_conf.set('g_threads_impl_def', 'WIN32')
   glib_conf.set('THREADS_WIN32', 1)
 else
+  # FIXME: probably needs more tweaking in meson for things like -D_REENTRANT etc.
+  thread_dep = dependency('threads')
+  threads_implementation = 'posix'
   pthread_prefix = '''
       #ifndef _GNU_SOURCE
       # define _GNU_SOURCE

meson_options.txt

@@ -68,3 +68,8 @@ option('bsymbolic_functions',
        type : 'boolean',
        value : true,
        description : 'link with -Bsymbolic-functions if supported')
+
+option('force_posix_threads',
+       type : 'boolean',
+       value : false,
+       description : 'Also use posix threads in case the platform defaults to another implementation (on Windows for example)')