Porting advice for gcc 10:
A common mistake in C is omitting <code>extern</code> when declaring a global
variable in a header file. If the header is included by several files it
results in multiple definitions of the same variable. In previous GCC versions
this error is ignored. GCC 10 defaults to <code>-fno-common</code>, which
means a linker error will now be reported. To fix this, use <code>extern</code>
in header files when declaring global variables, and ensure each global is
defined in exactly one C file. As a workaround, legacy C code can be compiled
with -fcommon.
This was not honored by the new ompipe module, because it is a local
file directive (it was applied to pipes as a side-effect of using the
same module for pipes and files...). I now made this a global, so that
semantics are the same as previously. Not really nice, but probably
the best thing to do in the current situation (everything else would
involve much more overhead --- leave that for the new config system).
