Stringify ICON_FILE, like other values passed in via -Dxx=yy

With an out of tree build like cmake, certain paths to the source tree and icon
file can foil parsing of this line by the resource compiler unless the path is
quoted.

Submitted by: Bert Huijben
Reviewed by: trawick

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1543282 13f79535-47bb-0310-9956-ffa450edef68