build: change how some OS specific source files are selected

In a bunch of cases, we emulate highly platform specific APIs on a
higher level across all OSes, such as IPC, terminal, subprocess
handling, and more. We have source files for each OS, and they implement
all the same mpv internal API.

Selecting which source file to use on an OS can be tricky, because there
is partially overlapping and emulated APIs (consider Cygwin on Windows).
Add a pick_first_matching_dep() function to make this slightly easier
and more structured.

Also add dummy backends in some cases, to deal with APIs not being
available.

Clarify the Windows dependency identifiers, as these are the most
confusing.

1 files changed, 23 insertions, 10 deletions

diff --git a/wscript b/wscript
index 58d64837af..7ac4982da4 100644
--- a/wscript
+++ b/wscript
@@ -12,6 +12,18 @@ from waftools.checks.custom import *
 c_preproc.go_absolute=True # enable system folders
 c_preproc.standard_includes.append('/usr/local/include')
 
+"""
+Dependency identifiers (for win32 vs. Unix):
+    wscript / C source      meaning
+    --------------------------------------------------------------------------
+    posix / HAVE_POSIX:                 defined on Linux, OSX, Cygwin
+                                        (Cygwin emulates POSIX APIs on Windows)
+    mingw / __MINGW32__:                defined if posix is not defined
+                                        (Windows without Cygwin)
+    os-win32 / _WIN32:                  defined if basic windows.h API is available
+    win32-desktop / HAVE_WIN32_DESKTOP: defined if desktop windows.h API is available
+"""
+
 build_options = [
     {
         'name': '--cplayer',
@@ -132,15 +144,15 @@ main_dependencies = [
         'fmsg': 'Unable to find either POSIX or MinGW-w64 environment, ' \
                 'or compiler does not work.',
     }, {
-        'name': 'win32',
-        'desc': 'win32',
+        'name': 'win32-desktop',
+        'desc': 'win32 desktop APIs',
         'deps_any': [ 'os-win32', 'os-cygwin' ],
         'func': check_cc(lib=['winmm', 'gdi32', 'ole32', 'uuid', 'avrt', 'dwmapi']),
     }, {
         'name': '--win32-internal-pthreads',
         'desc': 'internal pthread wrapper for win32 (Vista+)',
         'deps_neg': [ 'posix' ],
-        'deps': [ 'win32' ],
+        'deps': [ 'os-win32' ],
         'func': check_true,
     }, {
         'name': 'pthreads',
@@ -203,10 +215,11 @@ iconv support use --disable-iconv.",
             'posix_spawnp(0,0,0,0,0,0); kill(0,0)'),
         'deps_neg': ['mingw'],
     }, {
-        'name': 'subprocess',
-        'desc': 'posix_spawnp() or MinGW',
+        'name': 'win32-pipes',
+        'desc': 'Windows pipe support',
         'func': check_true,
-        'deps_any': ['posix-spawn', 'mingw'],
+        'deps': [ 'win32-desktop' ],
+        'deps_neg': [ 'posix' ],
     }, {
         'name': 'glob-win32',
         'desc': 'glob() win32 replacement',
@@ -506,7 +519,7 @@ audio_output_features = [
     }, {
         'name': '--wasapi',
         'desc': 'WASAPI audio output',
-        'deps': ['win32'],
+        'deps': ['win32-desktop'],
         'func': check_cc(fragment=load_fragment('wasapi.c')),
     }
 ]
@@ -583,7 +596,7 @@ video_output_features = [
     } , {
         'name': '--gl-win32',
         'desc': 'OpenGL Win32 Backend',
-        'deps': [ 'win32' ],
+        'deps': [ 'win32-desktop' ],
         'groups': [ 'gl' ],
         'func': check_statement('windows.h', 'wglCreateContext(0)',
                                 lib='opengl32')
@@ -671,7 +684,7 @@ video_output_features = [
     }, {
         'name': '--direct3d',
         'desc': 'Direct3D support',
-        'deps': [ 'win32' ],
+        'deps': [ 'win32-desktop' ],
         'func': check_cc(header_name='d3d9.h'),
     }, {
         'name': '--android',
@@ -774,7 +787,7 @@ hwaccel_features = [
     }, {
         'name': '--d3d-hwaccel',
         'desc': 'DXVA2 and D3D11VA hwaccel',
-        'deps': [ 'win32' ],
+        'deps': [ 'win32-desktop' ],
         'func': check_true,
     }, {
         'name': '--d3d-hwaccel-new',