The qgpgme source is missing 2 function declarations when built under Mac OS: for
strdup() and for strcasecmp().

The former can be addressed by invoking qstrdup() instead, which avoids setting
_DARWIN_C_SOURCE to a value which may or may not cause side-effects
(https://www.mail-archive.com/gnupg-users@gnupg.org/msg31883.html).

The latter declaration is obtained by including strings.h.