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
Merge pull request #121 from regehr/sclang-fix

sometimes clang/clang++ is used simply as an assembler or linker: no C/C...
Move a test to RoundTrip

RoundTrip supports examining expected output. We don't
need to create a separate test for this purpose.
Merge pull request #127 from chenyang78/parser_test

Move a test to RoundTrip