toplevel: build system continued

- toolchain
- make -n
- jobserver fix
- pkgconfig optional

toplevel build system continued
singular: remove already applied patches
singular: sanitize sanitize_gmp_header_hack
sagelib: spkg-install
sagelib: build system, continued
sage-doc dist
toplevel: move package description out of /build

"build" will be used to build packages.
toplevel  bsc
sagelib: bsc
python-sage: Makefiles FIXME
pytonbsc
toplevel bsc
python-sage: bsc
flintqs: fix patching path
pari: use --with-gmp-includedir
python-sage: makefiles
python-sage: libgap I