Avoid spawning an extra subshell for every header file.

Protect the environment elsewhere by putting the whole
header loop in a subshell. Noticed by Ralf Engelschall


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@84298 13f79535-47bb-0310-9956-ffa450edef68