[gtk-vnc] build: improve --with-coroutine=auto choice

[Daniel P. Berrange <dberrange src gnome org>]

commit e540fbc34d6bbfe3e84f1d9c33deaacc2326c7c9
Author: Jakub Janků <jjanku redhat com>
Date:   Mon Apr 26 17:38:13 2021 +0200

    build: improve --with-coroutine=auto choice
    
    When the required ucontext functions aren't found, switch to gthread
    coroutines instead of failing during the build.
    
    These functions are absent from the POSIX.1-2008.
    
    Fixes: https://gitlab.gnome.org/GNOME/gtk-vnc/-/issues/11
    
    Signed-off-by: Jakub Janků <jjanku redhat com>

 meson.build | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)
---
diff --git a/meson.build b/meson.build
index 8052d43..e9829e6 100644
--- a/meson.build
+++ b/meson.build
@@ -43,8 +43,20 @@ cc = meson.get_compiler('c')
 
 coroutine = get_option('with-coroutine')
 
+uctx_args = []
+if host_machine.system() == 'darwin'
+  uctx_args += ['-D_XOPEN_SOURCE=600']
+endif
+has_ucontext = true
+foreach uctx_func : ['makecontext', 'swapcontext', 'getcontext']
+  if not cc.has_function(uctx_func, prefix: '#include <ucontext.h>', args: uctx_args)
+    has_ucontext = false
+    break
+  endif
+endforeach
+
 if coroutine == 'auto'
-  if host_machine.system() == 'windows'
+  if host_machine.system() == 'windows' or not has_ucontext
     coroutine = 'gthread'
   else
     coroutine = 'ucontext'
@@ -52,6 +64,9 @@ if coroutine == 'auto'
 endif
 
 if coroutine == 'ucontext'
+  if not has_ucontext
+    error('ucontext requested, but not available')
+  endif
   conf_data.set('WITH_UCONTEXT', true)
 elif coroutine != 'gthread'
   error('Unknown coroutine backend')