toolchain/58399: build.sh fails in confusing way if MKX11=yes and xsrc/ exists but isn't mounted

[campbell+netbsd%mumble.net@localhost]

>Number:         58399
>Category:       toolchain
>Synopsis:       build.sh fails in confusing way if MKX11=yes and xsrc/ exists but isn't mounted
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    toolchain-manager
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Jul 05 00:30:00 +0000 2024
>Originator:     Taylor R Campbell
>Release:        current, 10, 9, ...
>Organization:
The NetBSD.sh -x Foundation
>Environment:
>Description:
If you have an xsrc directory, like /usr/xsrc, but it's a mount point for a file system and the file system isn't mounted, build.sh with MKX11=yes gets halfway through tools before failing in a very confusing way:

nbmake: don't know how to make makestrs.c. Stop
>How-To-Repeat:
1. create a /usr/xsrc mount point but forget to mount it
2. build.sh with MKX11=yes (or -x)
>Fix:
Teach build.sh to test for a file in xsrc, not just for an xsrc directory, early on.