CFLAGS only for clang in mixed-compiler project?

[Lev Serebryakov]

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 I'm trying to update arm-eabi (microcontroller) cross-gcc port to
latest version 4.9 and have one weird problem.

 Some part of gcc for arm (neon coprocessor machine description, to be
precise) requires more than 256 nested parenthesis in version 4.9 (4.8
doesn't have this problem). Due to this parenthesis madness clang
needs "-fbracket-depth=1024" option. If I add this option to CFLAGS in
environment variable, I have other problem. Later in build process gcc
uses newly-built gcc (xgcc) to build library. And this gcc picks up
"-fbracket-depth=1024" from environment and fails due to unknown option!

 How could I provide options only for clang but not for gcc?

- -- 
// Lev Serebryakov AKA Black Lion
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