Compiler-specific flags not used in Bisect

[mikebentley15]

Bug Report

Describe the problem
We have added compiler-specific flags with issue #125 (and associated pull request #301). These compiler-specific flags are in flit-config.toml. However, this was only done for the main generated Makefile, but not for Bisect Makefiles.

When running Bisect, the baseline compilation continues to use the compiler-specific flags. However, the compilation under test does not.

Suggested Fix
If the compiler specified in the compilation under test is specified in flit-config.toml, then take the compiler flags from there for generating object files.

But, if the compiler specified in the compilation under test is not found in flit-config.toml, then continue as we do now.

Question:
The links step is still done with the baseline compilation, so the link flags for the baseline compiler will be used. Should we do anything with the compiler-specific link flags for the compilation under test?

Alternative approaches:
I can't think of any. It seems a bit ridiculous to require the user to specify the fixed flags in the bisect command-line as well as in flit-config.toml. Think for instance if it is the same compiler as the baseline, but with a different optimization level and a different set of switches.