Makefile: don't require python-config for non-Python builds

Disable the Python shared library rules completely.  This avoids bogus
build error messages like this one:

 make: python-config: Command not found
Revert "DebugReplay: always set pressure altitude"

This reverts commit e8dec273cc4fe0bc96aba0c92dc93a91a4632828.

Seems like the IGC file spec gets overruled by the FAI Sporing Code:

FAI Sporting Code and Gliding Rules Section 3:
Gliding Edition 2013 /Appendix C / Paragraph 6.2

6.2 PR file format and testing because PRs are simpler than flight
recorders, some non-vital data fields may not be present. Pressure
altitude in the .igc file is to be recorded as zero unless it is
derived from a pressure sensor calibrated to the ICAO Standard
Atmosphere. The tests described below should be shown in the PR
files, and files from an FR should be included for comparison.