PKG_CONFIG_PATH gets overridden during post_install
This was picked up when building the qt5 variant with an external SDK.
This variant requires PKG_CONFIG_PATH to be set to the relevant Qt*.pc files. Even though building succeeds, post_install fails with the following:
Package Qt5Core was not found in the pkg-config search path.
Perhaps you should add the directory containing `Qt5Core.pc'
to the PKG_CONFIG_PATH environment variable
Package 'Qt5Core', required by 'gstqmlgl', not found
The gstqmlgl.pc file makes reference to Qt5Core and friends, which become lost when PKG_CONFIG_PATH gets overridden during the post_install phase.
A workaround is to create symlinks to the .pc files in one of the built-in search paths such as /usr/share/pkgconfig or /usr/lib/pkgconfig.
My suggestion would be to allow for an override, or to rather merge the env.