meson: Compile libyaml as a subproject if not available on the system

AOSP doesn't ship libyaml, making it more difficult to compile libcamera
for Android. Use a meson wrap to compile libyaml as a subproject if the
dependency is not found.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Eric Curtin <ecurtin@redhat.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Umang Jain <umang.jain@ideasonboard.com>

src/libcamera/meson.build

@@ -67,7 +67,7 @@ subdir('proxy')
 libdl = cc.find_library('dl')
 libgnutls = cc.find_library('gnutls', required : true)
 libudev = dependency('libudev', required : false)
-libyaml = dependency('yaml-0.1', required : true)
+libyaml = dependency('yaml-0.1', required : false)
 
 if libgnutls.found()
     config_h.set('HAVE_GNUTLS', 1)
@@ -88,6 +88,17 @@ if libudev.found()
     ])
 endif
 
+# Fallback to a subproject if libyaml isn't found, as it's not packaged in AOSP.
+if not libyaml.found()
+    cmake = import('cmake')
+
+    libyaml_vars = cmake.subproject_options()
+    libyaml_vars.add_cmake_defines({'CMAKE_POSITION_INDEPENDENT_CODE': 'ON'})
+    libyaml_vars.append_compile_args('c', '-Wno-unused-value')
+    libyaml_wrap = cmake.subproject('libyaml', options : libyaml_vars)
+    libyaml = libyaml_wrap.dependency('yaml')
+endif
+
 control_sources = []
 
 foreach source : control_source_files