fixed a C compiler warning on FreeBSD introduced in the previous commit. 

bugfix: fixed a compiler warning on type mismatch when dtrace USDT is… … enabled in the build.

bugfix: fixed compiler warnings "comparison between signed and unsign… …ed integer expressions" on Windows.

bugfix: fixed a potential data alignment issue in the ngx.var setter … …API.

bumped version to 0.9.19. 

doc: bumped version to 0.9.18. 

bumped the API version to 0.9.18. 

doc: bumped version to 0.9.17. 

bugfix: the user specified ./configure's --with-cc-opt and --with-ld-… …opt might override the LUAJIT_INC/LUAJIT_LIB and LUA_INC/LUA_LIB environment settings. thanks Julian Gonggrijp for the report in openresty/openresty#117.

bugfix: we might miss the linker option -ldl when we shouldn't.