On macOS with Homebrew's argp-standalone, Findargp's link test for
argp_parse fails with an undefined strchrnul, because libargp.a itself
references that GNU extension and the platform has none. The module then
stops with "does not have a symbol named argp_parse", which is not what
is wrong.
Probe for strchrnul first (with _GNU_SOURCE, so glibc answers correctly).
Where it exists, the test is unchanged. Where it is missing, run the same
argp_parse test with a local definition of strchrnul, and have ptouch-print
supply that definition for the real build.
On glibc nothing changes: the probe finds strchrnul and the original test
runs. Checked on Debian stable in the same session.
pkg_check_modules gives both LIBUSB_LIBRARIES (bare names, -lusb-1.0) and
LIBUSB_LINK_LIBRARIES (absolute paths). Passing both links libusb twice,
and the bare name only resolves when libusb is in the default library
search path - which it is on Linux and is not under Homebrew on macOS,
where the link fails with "library not found for -lusb-1.0". The full
path alone is enough everywhere.
Kevin Bradenstein