Allow to control LIBDIR

Some distributions have lib32/lib64 and need to control
the destination properly.
Quote variables that could contain whitespaces

Make does NOT take care of this in this context. It's
not a variable assignment and installation would fail
if DESTDIR contains any whitespace.
Respect CFLAGS/CXXFLAGS/LDFLAGS, fixes #66

Even in linking command CFLAGS should be respected, no matter
if all objects have already been compiled or not. Some flags
affect both linking and object compilation and users shouldn't
be required to figure them out. This is standard in most build
systems anyway.

Reorderd the linker line a bit, so it matches common practice,
although this doesn't matter technically (except that LDFLAGS
must go before libraries we link against).
Merge branch 'trunk' of github.com:wbhart/flint2 into trunk
Fix NTL test build
Exclude "%templates" dirs from extensions.
Merge remote-tracking branch 'hart/trunk' into trunk
Fix back test for EXT_HEADERS value.
Merge branch 'trunk' of https://github.com/hasufell/flint2 into trunk

Conflicts:
	Makefile.in
Merge branch 'trunk' of https://github.com/jpflori/flint2 into trunk

Conflicts:
	Makefile.in