Configure: fix gcc version detection in some corner cases

Configure: fix gcc version detection in some corner cases

If the "gcc version ... " string appeared within "Configured with:", it
was picked up rather than the real gcc version string. This might then
break the configure scripts due to a malformed NGX_COMPILER macro.

The simple fix is to look for "gcc version ... " at the start of the
line, rather than anywhere within.

Suggested-by: Aleksei Bavshin <a.bavshin@nginx.com>
Closes: https://github.com/nginx/nginx/issues/1278