This page is linked from the build system. Do not remove it without making sure it links to useful information for building Firefox 3
wpcapi.h is a header file to interact with Windows Vista parental controls.
The error "System header wpcapi.h is not available" indicates that your copy of the Windows SDK is not new enough. There are several ways you can solve this problem:
Install the Windows Vista or Windows 2008 Server SDK
Installing a new SDK will provide the wpcapi.h header file. See the Windows Build Prerequisites for details.
The new SDKs currently don't work well with the free (express) versions of Visual C++. MozillaBuild 1.3 allows you to use both a new SDK and an older Platform SDK together to avoid this problem.
Installing the SDK doesn't remove the build cache so if you've previously built without it, after installing the SDK you will need to manually remove config.cache from your build.
Disable Vista SDK Requirements
The following flag disables Mozilla features which depend on Microsoft SDK versions 6.0 and higher. You can add this flag to your mozconfig file: