* gnu/packages/gcc.scm (gcc-4.7)[arguments]: Adjust configure-flags to
build gcc-4.8 using the C++03 standard.
(gcc-4.8)[arguments]: Add a phase to remove the current gcc's C++
headers from the CPLUS_INCLUDE_PATH.
(gcc-4.9)[arguments]: Inherit from gcc-4.8.
(gcc-6)[arguments]: Inherit from gcc-4.7.
Change-Id: Ibec0683e8385ae011e6a809c664cb679f1a35b80