Clang reports itself as GCC 4.2.1 when asked the GCC way, which is about
as good an answer as any, since there's no good mapping between them.
So when testing GCC version, Clang would be usually considered too old,
and therefore the proper way is to do a configure check if possible.
GCC version should be only used for GCC-specific things such as avoiding
a bug in a specific GCC version, and such testing should first check
the compiler is not Clang.
Rename GCCVER to GCC_VERSION and use it throughout the build system
where needed. As it's empty for anything that's not GCC, this should
lead to errors when used incorrectly.

Change-Id: Iea96bbaf5d8ceabefa25be88576eeb4115384937