ghc-toolchain inconsistency on Centos 7

The Centos 7 nightly job currently fails with:

configure:18352: WARNING:
There are some differences between the toolchain configured by "configure" (hadrian/cfg/default.target) and the toolchain configured by the "ghc-toolchain" program (hadrian/cfg/default.target.ghc-toolchain).
20c20
< , tgtCmmCPreprocessor = CmmCpp {cmmCppProgram = Program { prgPath =  "/usr/bin/gcc" , prgFlags = ["-E"] }, cmmCppSupportsG0 = True}
---
> , tgtCmmCPreprocessor = CmmCpp {cmmCppProgram = Program { prgPath =  "/usr/bin/gcc" , prgFlags = ["-std=gnu99","-E"] }, cmmCppSupportsG0 = True}