Allow kernel extensions to use GAP header files via #include <gap/HEADER.h> instead of #include "src/HEADER.h" or #include "HEADER.h" (improves compatibility with "install" GAP versions shipped by Linux distributions)

[fingolfin]

Right now, some GAP packages still use #include "src/HEADER.h" to
access GAP headers. Typically one sees #include "src/compiled.h" to
get "all" (well, most) GAP headers in one go.

That's not great for various reasons. For one, it is not obvious that
this is a GAP header (this is often dealt with by adding a comment). It
also doesn't mesh well with GAP installations made via make install,
which install headers into PREFIX/include/gap.

Indeed, we have a workaround in place specifically to deal with that,
which installs a file PREFIX/include/gap/src/compiled.h that includes
the real compiled.h to allow such packages to compile anyway. But
that's not quite enough as some packages directly include some other
headers (e.g. because compiled.h does not include them). To deal with
that better, this patch now installs a second set of the headers into a
src directory so that all those packages work again.

Moreover, compiled.h (or rather gap_all.h) is extended to cover two
more headers used by packages.

The longterm plan is to migrate packages away from this. Indeed, I
already changed many packages to at least use #include "compiled.h"
with the goal to eventually dispose of the src hack altogether.

Going one step further, the plan is that one day packages will be able
to use #include <gap/HEADER.h>. This PR enables this: for classic GAP
builds, it does so by creating a symlink build/gap pointing to src.
And for make install we modify the GAP_CPPFLAGS in the installed
sysinfo.gap suitably.

In addition, already today, packages can do #include <gap_all.h>,
provided one is willing to require at least GAP 4.11.1.

[fingolfin]

Backported to stable-4.12 in commit 183d2e5