build: use distributed man pages when running with --help fails

build: use distributed man pages when running with --help fails

When building against an incompatible GLIBC version compared to that
on the build host, then running the just-built binary might fail
although it is the same platform - thus CROSS_COMPILING is false.
As a result, generating the man pages fails.

* man/local.mk (.x.1): Add a check to verify that running the utility
with --help succeeds, otherwise falling back to using 'dummy-man'.