Add explicit C compiler support (issue #337)

[drs546nau]

Closes issue #337

• Added COMPILER_C key to EXECUTABLES in afl.py
• Added compiler_c_exe attribute initialized from EXECUTABLES in base.py
• Resolved compiler_c_exe in _set_executables() in fuzz.py
• Detect .c vs .cpp in compile() and select correct compiler and -std flag
• Falls back to stripping ++ from compiler_exe when COMPILER_C
• Added unit tests in tests/test_compiler_c_exe.py

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