Change ${CMAKE_BINARY_DIR} to ${CMAKE_CURRENT_BINARY_DIR}

[ds283]

• if building as a submodule of another repository, installation of pkg-config files can fail because they may not be in the top-level binary directory
• this seems to happen because the CMake INSTALL command references ${CMAKE_BINARY_DIR}
• changing the INSTALL command to use ${CMAKE_CURRENT_BINARY_DIR} allows CMake to find the files for installation

[cdunn2001]

LGTM, but I need another pair of eyes on this.

[rpavlik]

Yes, that is more correct than the previous code. The configure file doing the generation is right above and uses a relative output path, and per https://cmake.org/cmake/help/v3.0/command/configure_file.html relative output paths are interpreted with respect to the current binary directory.

[cdunn2001]

Thanks, Ryan! (@rpavlik)