sometimes clang/clang++ is used simply as an assembler or linker: no C/C++ files
are given on the command line. in this case it is unhappy about the unused
Souper arguments and prints unsightly warnings.  some build systems take these
as errors.  this patch detects whether any C/C++ files are being compiled and,
if not, doesn't give the problematic Souper arguments.

also: trivial patch to pass the -souper-ignore-solver-errors to souper if asked
by an environment variable