gmc-mh-1x-2x: Fixed 300 µA range scaling.
gmc-mh-1x-2x: Cosmetic fixes for Metrahit 25S.
visa: Fixed missing header warnings/uninitialized variable.
gmc-mh-1x-2x: Whitespace fixes (cosmetic).
brymen-bm86x: disable driver when libusb is not present
Fix 'aquisition' typo in a few places.
scpi_usbtmc_libusb.c: Consistently check for < 0 (libusb calls).
scpi_usbtmc_libusb.c: Fix two error checks.

Two error checks had a missing "ret = ", which lead to an incorrect
value being passed to libusb_error_name().

Also, lower the level for those messages from sr_err() to sr_dbg()
since they're not fatal.
Switch to a non-recursive automake setup.

Instead of >= 44 Makefile.am's we now only have one top-level
Makefile.am, and use the 'subdir-objects' automake option to
handle the build via non-recursive (auto)make.

This has the advantage of fewer (boilerplate or other) files and less
clutter in general, as well as performance advantages since the new
setup can build many files in parallel (with 'make -j'), not only 2 or 3
files within the same (e.g. hardware/xxxx/* subdirectory) and also since
we no longer need to build intermediate libtool helper libs per subdirectory.

A quick, non-scientific test build on a quad-core laptop with 'make -j 4'
yields a build time reduction from 35s to 19s.

All autotools features that worked before are still intact without any
regressions, including the Make targets 'install', 'uninstall', 'check',
'dist', 'clean', 'distclean' and so on, as well as all the usual portability
handling (build works on any OS, with any Make implementation such as
GNU Make or BSD Make, with any shell such as sh/ksh/zsh/bash/dash, etc. etc.)
and features such as out-of-tree build support, cross-compile support,
testsuite support (also with colored output), "silent make rules", etc. etc.
Makefile.am: Add missing vxi.h.

If it's not listed as SOURCE, it won't end up in the tarball and a build
from tarball would thus break.