prevent icerun-wrapped commands failing if path contains 'clang'

'icerun /path/contains/clang/some_command' would make compiler_is_clang()
return true, because analyze_program() keeps the path in compilerName()
for Lang_Custom CompileJob , unlike the other CompileJob kinds, which
store the compiler name without a path. This problem would show when
running this command, because analyse_argv() based on this would try
to add -fcolor-diagnostics to the command arguments in some circumstances.