dev-libs/appstream-0.12.6 with use flags "apt introspection qt5 -test" fails to build, with a bunch of errors like

ror:template with C linkage， note: ‘extern "C"’ linkage started here 

Looking at appstream repo, this is related to newer version of glib (which we use) and was solved with this commit https://github.com/ximion/appstream/commit/8a179f14619103247e4a14d96ea03c358d9c1492#diff-9d75ab828fb7ece246975ea3f8415abff413919ae80906b5573d45dd2ca84c37 in version 0.15.4

I managed to get appstream to compile on my machine by providing a user patch file, so we can either:

• upgrade appstream version
• use the patch file appstream-patch.tar.gz