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>

subprojects/.gitignore

@@ -1,4 +1,5 @@
 /googletest-release*
+/libyaml
 /libyuv
 /packagecache
 /pybind11

subprojects/libyaml.wrap

@@ -0,0 +1,5 @@
+[wrap-git]
+directory = libyaml
+url = https://github.com/yaml/libyaml
+# tags/0.2.5
+revision = 2c891fc7a770e8ba2fec34fc6b545c672beb37e6