Improve cross-compile support

Improve cross-compile support

The check for the signedness of size_t really doesn't have to be run as
that is a compile time property.

Beyond that, dovecot uses mysql_config. I've looked into that and
mysql_config is unfixably broken during cross builds. It will not be
fixed. Instead, please use pkg-config. My patch implements that with a
fallback to mysql_config to avoid breaking other users.

Last but not least, src/lib-lua/Makefile.am adds $(LUA_LIBS) to
libdovecot_lua_la_DEPENDENCIES. As it happens, LUA_LIBS contains a -L
flag and when that flag shows up in a dependency, make gives up. I have
no clue why one would add LUA_LIBS to DEPENDENCIES as it already is
being correctly added to LIBADD. My patch suggests to quite simply drop
that.

Gbp-Pq: Name Improve-cross-compile-support.patch