Store actual tarball name in package metadata.
Use stored tarball name for downloading.
Merge branch 'master' into ticket/14968

Conflicts:
	build/pkgs/sagenb/checksums.ini
switched the large Cremona ellcurve database to the new git directory layout
adding first files database_symbolic_data to build/pkgs
updated libogg and libtheora, fixed sage-git layout
add fixed openssl
added more requisites for git layout
added the topcom optional spkg
do not suggest force push
build git without i18n if gettext is not available
Merge branch 'master' into 14968

Conflicts:
	build/pkgs/boehm_gc/checksums.ini
	build/pkgs/ipython/checksums.ini
	build/pkgs/pari/checksums.ini
	build/pkgs/ppl/checksums.ini
	build/pkgs/sagenb/checksums.ini
	build/pkgs/sympy/checksums.ini
	src/bin/sage-spkg
Allow VERSION variable in spkg tarball specification.
Add missing tarballs, use VERSION in tarball path.
Use explicit date for git doctests.
specify environment explicitly for doctests

the git interface seems to ignore changes made to os.environ
also fix the install target for git without gettext
fix ppl tarball
Enable cython cache.
fix checksum regex for POSIX checksums