enable building with CCE

[JorgeG94]

I had to unset a variable in order for the code to build with CCE otherwise I could not build.

[cgmb]

Thanks for the contribution. The SET_COMPILE_FLAG function does seem to be a pain point for supporting alternative compilers. I suspect that your change may have side-effects that would negatively affect the builds for other compilers, but I don't fully understand the surrounding code so I cannot say for sure yet. Once I understand the surrounding context, I'll give you a firm thumbs up or thumbs down on merging this PR.

[bcornille]

It may be worthwhile to borrow the design of blt_append_custom_compiler_flag to make SET_COMPILER_FLAG more robust.