Improve GNU make based configure “script”

[sebschrader]

A collection of improvement to the GNU make based configure “script” configure.mk:

• Usage outside of the Debian package build environment:
  • Find Postgres programs without Debian PgCommon perl module
  • Don't try to find programs or invoke the shell, if a substitution variable has already been defined by the user (e.g. env var or CLI var).
• Correctness/Troubleshooting
  • Always check the exit status when using the shell function (GNU Make .SHELLSTATUS).
  • Don't expand undefined optional parameters
• Performance
  • Find programs directly with make (without shell)