MDEV-23926: Fix warnings generated during compilation of plugin/auth_pam/mapper/pam_user_map.c on MacOS

Compiler warnings like one listed below are generated during server build on MacOS:

[88%] Building C object plugin/auth_pam/CMakeFiles/pam_user_map.dir/mapper/pam_user_map.c.o
mariadb/server-10.2/plugin/auth_pam/mapper/pam_user_map.c:87:41: error: passing
'gid_t *' (aka 'unsigned int *') to parameter of type 'int *' converts between pointers to integer types
with different sign [-Werror,-Wpointer-sign]
  if (getgrouplist(user, user_group_id, loc_groups, &ng) < 0)
     ^~~~~~~~~~
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk/usr/include/unistd.h:650:43: note:
  passing argument to parameter here
  int getgrouplist(const char *, int, int *, int *);
      ^

In case MariaDB server is build with -DCMAKE_BUILD_TYPE=Debug it results in
build error.

The reason of compiler warnings is that declaration of the Posix C API function
getgrouplist() on MacOS differs from declaration of getgrouplist() proposed
by Posix.

To suppress this compiler warning cmake configure was adapted to detect what
kind of getgrouplist() function is declared on the build platform and
set the macros HAVE_POSIX_GETGROUPLIST in case the building platform supports
Posix compatible interface for the getgrouplist() function. Depending on
whether this macros is set the compatible type of arguments is used to pass
parameter values to the function.