pkgconfig module: correctly generate Libs search path with absolute install_dir
pkgconfig module: correctly generate Libs search path with absolute install_dir

For example the OpenRC build files install libraries to install_dir: '/lib'
and this works, but causes the generated pkg-config to say:

prefix=/usr

Libs: -L${prefix}//lib

which is both ugly (double //) and resolves to /usr/lib which is exactly
what does not work.

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