cmake: define STRERROR_R_CHAR_P for GNU-specific strerror_r

cmake: define STRERROR_R_CHAR_P for GNU-specific strerror_r

This is what autotools does with AC_FUNC_STRERROR_R bit.  It takes us
from

    $ monmaptool --print nonexistant
    monmaptool: monmap file nonexistant
    monmaptool: couldn't open nonexistant: (2)

to

    $ monmaptool --print nonexistant
    monmaptool: monmap file nonexistant
    monmaptool: couldn't open nonexistant: (2) No such file or directory

on cmake - otherwise we end up using GNU-specific version as if it was
the POSIX version.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>