To build a .zip or .tar.gz/.tar.bz2 package, just go to xpinstall/packager (in the objdir, if any) and type:
make
This will select a default package format, depending on the OS.
To specify a specific package format, use
make MOZ_PKG_FORMAT=format
where format is one of DMG (MacOS only), BZ2, TGZ and ZIP.
The resulting package is placed in the dist/ directory.
On platforms that support it, you can just go to xpinstall/packager (again, in the objdir if you use one) and type
make installer
This will do nothing on platforms that have no installer.
The installer will be placed in dist/install/sea and dist/install/stub (windows) or installer/{stub,sea} (linux).
msvcrt.dll and msvcirt.dll: These files must be copied to the %MOZ_SRC%\redist\microsoft\system directory on your build system. Without these files, the build will not fail, but the package you create will have problems on rare Windows systems that don't already have the files installed (mostly Win95) as described in bug 27601.