lrzsz: fix compilation with GCC14

lrzsz: fix compilation with GCC14

Trying to compile with GCC14 will fail on compiler sanity check with:
configure:1056:1: error: return type defaults to 'int' [-Wimplicit-int]
 1056 | main(){return(0);}
      | ^~~~

This is due to GCC14 not allowing implicit integer types anymore[1].

So, patch configure to avoid this and make it compile with GCC14.

Proper fix would be to use autoreconf to rebuild configure but configure.in
is completely outdated and would likely be more broken when regenerated.

[1] https://gcc.gnu.org/gcc-14/porting_to.html#implicit-int

Signed-off-by: Robert Marko <redacted>