Avoid side-effects in src/Configure when the user exports the shell

variables via

$ CFLAGS=...
$ export CFLAGS
$ ./configure

(which is not needed) instead of

$ CFLAGS=.. ./configure

PR: 2101


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@80999 13f79535-47bb-0310-9956-ffa450edef68