Trac #33046: gap spkg creates absolute symlinks

In `gap/spkg-install` two absolute symlinks are created; this confuses
packagers and relocation.

The current ticket replaces them by relative symlinks.

All the paths involved are hardcoded in this file (even `SAGE_BIN`), so
nothing external can affect it.

URL: https://trac.sagemath.org/33046
Reported by: tornaria
Ticket author(s): Gonzalo Tornaría
Reviewer(s): Matthias Koeppe

Author: Release Manager

build/pkgs/gap/spkg-install.in

@@ -80,8 +80,8 @@ chmod +x "$SAGE_DESTDIR$SAGE_BIN/gap"
 
 # Create symlinks under $GAP_ROOT for these executables, as they are expected
 # (especially when building kernel packages) to exist
-ln -sf "$SAGE_BIN/gap-bin" "$DESTDIR_GAP_ROOT/gap"
-ln -sf "$SAGE_BIN/gac" "$DESTDIR_GAP_ROOT/gac"
+ln -sf "../../bin/gap-bin" "$DESTDIR_GAP_ROOT/gap"
+ln -sf "../../bin/gac" "$DESTDIR_GAP_ROOT/gac"
 
 # Fix the $GAP_ROOT/bin/<arch>/src symlink to be relative (otherwise it links
 # to the actual path of the sources GAP was compiled from)