pkg-config allows a way to override package variables through the
--define-prefix interface, but this is very cumbersome to do in a global
way since it always needs to be passed on the command line and the
override cannot be scoped to a single packge.
Allow overriding package variables using environment variables of the
form PKG_CONFIG_$PACKAGE_$VARIABLE. For example, setting
PKG_CONFIG_GLADEUI_2_0_CATALOGDIR will override the variable
"catalogdir" in the "gladeui-2.0" package.
https://bugs.freedesktop.org/show_bug.cgi?id=90917
