Allow system package for `kannel-dev`

On Debian, I decided to not use the sources for Kannel as you suggest, but install kannel-dev instead. This did not work as expected, due to the autotools setup (which I cannot fix, being a CMake user).

Here's what I found, reporting in the hope it is useful to you.

Regular expressions. I did not find the gwlib/gw-regex.h. It's funny that it includes the file with double quotes instead of <kannel/gwlib/gew-regex.h>,

libsmpp/smpp_route.h:65:10: fatal error: gwlib/gw-regex.h: No such file or directory
   65 | #include "gwlib/gw-regex.h"

But, is this really Kannel functionality or should it be <regex.h> for the standard POSIX functionality? FWIW, the include file is not part in the OS package, whereas <regex.h> is available, along with regcomp() and regmatch() functions. (I tried brutally renaming with #define gw_regex_comp regcomp but it's a little more contrived, I got compiler errors and am sure a linker error would also follow).

Prototypes. Several prototypes for the library's own functions are undefined. Maybe include them in a header file? This impacts linker-friendliness for the library after all. Example:

   79 | sig_atomic_t smpp_bearerbox_get_state(SMPPBearerbox *smpp_bearerbox) {
      |              ^~~~~~~~~~~~~~~~~~~~~~~~
libsmpp/smpp_bearerbox.c:87:6: warning: no previous prototype for 'smpp_bearerbox_set_state' [-Wmissing-prototypes]

Redefines. You can call gcc paranaoid, but it complains about redefinitions of #define without prior #undef. It does that for PACKAGE, PACKAGE_BUGREPORT, PACKAGE_NAME, PACKAGE_STRING, PACKAGE_TARNAME, PACKAGE_URL, PACKAGE_VERSION and VERSION. Harmless warnings, but they clutter the build process and can easily be avoided with #undef. Example:

../config.h:44: warning: "PACKAGE" redefined
   44 | #define PACKAGE "ksmppd"
      | 
In file included from /usr/include/kannel/gwlib/gw-rwlock.h:66,
                 from /usr/include/kannel/gwlib/gwlib.h:104,
                 from ksmppd.c:70

kneodev / ksmppd

Allow system package for `kannel-dev` #157