RFC: Use gcc -dumpfullversion to get minor version with gcc-7.x
RFC: Use gcc -dumpfullversion to get minor version with gcc-7.x

In gcc-7.1, the behavior of -dumpversion changed to be configured
at compile-time. On some distributions it only dumps the major version
(e.g., Ubuntu), so the current checks for the gcc minor version report
false negatives. As a replacement, gcc-7.1 introduced -dumpfullversion
which always prints the full version.

Update the gcc version detection in Makefile.system to employ
-dumpfullversion with gcc-7 and newer.

Posting this patch for discussion, since it emerged from discussions
around issue #2668 and PR #2669. It is not solving a problem right now,
but may be useful in the future.

Signed-off-by: Marius Hillenbrand <fe1e41a2593d8a0d1128edc53cee7053b8cae934@linux.ibm.com>

xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS