Package rpm-build: Information

- Export FCFLAGS as modern FFLAGS replacement for gfortran.

- ldd.in: made preloading of PIE objects work again.
- Set the value of SOURCE_DATE_EPOCH environment variable (if any)
  as the source package buildtime.

- find-package, shebang.req: introduced RPM_FINDPACKAGE_MANDATORY=1.
  When an interpreter is invoked by name, as in "#!/usr/bin/env python32",
  and is missing, this will now force the dependency on /usr/bin/python32.