If _GNU_SOURCE is defined on linux, then strerror_r() is an alternate,
non-POSIX version.

Undefine _GNU_SOURCE when including <string.h> in ConfigManager.cpp to
get the POSIX version of strerror_r(), and initialize the error string
buffer to "unknown error" so that the code does not crash whichever
version of the library function is being used, or strerror_r() fails for
some reason.