gap headers and libgap headers can coexist if both are installed to their respective $pkgincludedir. libgap headers should go into $prefix/include/libgap.

The references to the headers need to be changed accordingly.

Depends on #14804

Upstream: Not yet reported upstream; Will do shortly.

CC: @jondo

Component: packages: standard

Author: Felix Salfelder

Branch/Commit: u/felixs/libgap_include @ 8bb4a27

Issue created by migration from https://trac.sagemath.org/ticket/15112