2geom.pc.in hardcodes "lib" for libdir
This is a forward of [Gentoo Linux bug 938483](https://bugs.gentoo.org/938483).
2geom.pc.in hard-codes `libdir=${exec_prefix}/lib`. On 64-bit systems, this should be `lib64` instead.
Presumably it should use `@CMAKE_INSTALL_LIBDIR@` (and `@CMAKE_INSTALL_INCLUDEDIR@` for `includedir`).
issue