Add 	-gcc-toolchain to list of INCLUDE_FLAGS

-gcc-toolchain expects a path to follow it.   Adding this flag to the
list of INCLUDE_FLAGS prevents YCM from stripping the argument to the
flag.

The environment that this was needed on was a RHEL6 system where the
desired compiler is gcc 4.8.1 which is installed in a non-default
location.  As a result,   YCM was using headers from gcc 4.4 instead of
gcc 4.8 which doesn’t work so well when using the -std=c++11 flag.