Link the module against the installed MEOS archive

[estebanzimanyi]

The emcc line takes MEOS from the install prefix, so the prefix is the only
thing it names and the build-tree layout stays MEOS's own business.

MEOS installs one self-contained archive: it folds pgtypes, liblwgeom and ryu
into libmeos.a — and libpc with POINTCLOUD on — so a consumer embedding MEOS in
a single relocatable module links one library. The build-tree paths the link
line used instead are intermediates that the install does not publish, and
meos/CMakeLists.txt places the archive at the binary root rather than under
build/meos, so build/meos/libmeos.a names a file that is not written and
emcc stops at the missing input. The sidecar libpgtypes.a and libpostgis.a go
with it, their objects already being inside the archive.

The source-tree include paths go too: the installed umbrella headers resolve
among themselves and reach outside the prefix only for json-c and h3, which
carry their own. Keeping the MobilityDB source on the include path let its
tracked pg_config_manual.h and postgres_ext.h answer ahead of the installed
copies, which is the shape that compiles against one set of definitions and
links against another.

The embedded spatial_ref_sys.csv comes from the prefix for the same reason.