If glib-2.0 is not present on macOS, binaries of the static library and
header files are downloaded from a GitHub repo. This prepares to get RcppCWB
pass macOS checking on CRAN machines.
A slight modification of the C code will now prevent previous crashes resulting
from a faulty CQP syntax. The solution will not yet be effective for Windows
systems until we have recompiled the libcqp static library that is downloaded
during the installation process.
A new C++-level function 'check_corpus' checks whether a given corpus is
available and is used by the check_corpus()-function. Problems with
the previous implementation that relied on files in the registry directory to
ensure the presence of a corpus hopefully do not occur.
Calling the 'find_readline.perl' utility script is omitted on macOS, so
previous warning messages when running the makefile do not show up any more.