build: accept gcc/g++ without minor version

build: accept gcc/g++ without minor version

Build dependency: Please install the GNU C Compiler (gcc) 4.8 or later cc
  -dumpversion | grep -E '(4\.[8-9]|5\.[0-9]|6\.[0-9]|7\.[0-9])'
Build dependency: Please install the GNU C++ Compiler (g++) 4.8 or later
  g++ -dumpversion | grep -E '(4\.[8-9]|5\.[0-9]|6\.[0-9]|7\.[0-9])'

Prerequisite check failed. Use FORCE=1 to override.

On my Fedora 26 machine gcc and g++ -dumpversion returns a whole number
'7' failing the regex introduced in commit:

b78de6207f6fc1a9db857942cb89f9fcf730a240

This change makes minor versions optional in the build dependency regex
for gcc and g++ whenever any minor version would be accepted and the
whole number version is sufficient as a dependency check. For versions
4.* a minor version is still required.

Signed-off-by: Justin Kilpatrick <jkilpatr@redhat.com>