What build flags are available for brew install of gcc?
Use brew options. On my machine I get this:

brew options gcc48
--enable-all-languages
        Enable all compilers and languages, except Ada
--enable-cxx
        Build the g++ compiler
--enable-fortran
        Build the gfortran compiler
--enable-java
        Buld the gcj compiler
--enable-multilib
        Build with multilib support
--enable-nls
        Build with native language support (localization)
--enable-objc 
        Enable Objective-C language support
--enable-objcxx
        Enable Objective-C++ language support
--enable-profiled-build
        Make use of profile guided optimization when bootstrapping GCC