saslplugin: Fix build under C99 on all platforms
C99 changes the meaning of extern inline (https://stackoverflow.com/questions/216510/what-does-extern-inline-do).
In our case that has the effect that some headers from glibc are treated
differently, i.e. functions like atof are actually defined and exported
in the object files including the header defining atof and friends.
More information about this issue: https://gcc.gnu.org/legacy-ml/gcc/2006-11/msg00011.html
Normally this should not be the case; the underlying issue is that libkgapi is using a generated config.h (./src/saslplugin/config.h) which is causing issues under C99.
Fix this by generating the config.h file using CMake.
This patch also updates the 3rdparty plugin_common.* to a more recent version (cyrus-sasl-2.1.27 to be exact), providing also support for "Windows 10 or greater".