Changes MACOSX_DEPLOYMENT_TARGET to 10.5

Installing the lsqlite3 package uses the -rpath option to gcc,
which is only valid with a MACOSX_DEPLOYMENT_TARGET of 10.5 or greater.

Without the change, trying `luarocks install lsqlite3` shows:
```
export MACOSX_DEPLOYMENT_TARGET=10.3; gcc -bundle -undefined dynamic_lookup -all_load -o lsqlite3.so -L/usr/local/lib lsqlite3.o -L/usr/local/lib -Wl,-rpath,/usr/local/lib: -lsqlite3
ld: -rpath can only be used when targeting Mac OS X 10.5 or later
```

With this change, lsqlite3 can be successfully installed on OSX.