install-deps.sh: point gcc to the one shipped by distro

install-deps.sh: point gcc to the one shipped by distro

to define a struct in a method is legal in C++11, but it causes internal
compiler error due to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82155
if we are using GCC-7. so we need to either workaround in our source
code by moving the struct definition out of the member method or revert
to a GCC without this bug. but if we go with the first route, the jewel
build still fails, because GCC-7 starts to use the new CXX11 ABI, which
is not compatible with the libboost we use in jewel. the libboost was
still built with the old ABI for backward compatibility. so let's just
fix the install-deps.sh to point gcc to the origin one.

See: http://tracker.ceph.com/issues/22220
Signed-off-by: Kefu Chai <kchai@redhat.com>
Conflicts: the libboost issue does not affect master. as master builds
 boost from source. so, it's not cherry-picked from master.