File structure fixes.
Ignore file rule cleanup.
General Makefile rule cleanup.
Add early termination if of Makefile rules if downloads fail.
Save Python environment for future use in the cache directory.
Fix incorrect string comparison (causing error messages on BSD builds).
Add GMP, MPFR, and MPC as compiled dependencies of the Python environment, as these libraries are used by gmpy2.
Upgrade Python virtualenv to 1.10.1.
Remove compiled libraries and temporary file listing from version control.
Upgrade project dependencies to latest versions.
Remove SQLite database file at default location from version control.