Automatically set `CPPFLAGS` and `LDFLAGS` on MinGW
Automatically set `CPPFLAGS` and `LDFLAGS` on MinGW

If PATH is set appropriately, it is unnecessary to use
`--with-opt-dir`, `--with-opt-lib`, and `--with-opt-include` options.

jonmartindell/rmagick,rmagick/rmagick,antonifs/rmagick,carsonreinke/rmagick,antonifs/rmagick,carsonreinke/rmagick,jonmartindell/rmagick,carsonreinke/rmagick,tvon/rmagick,jonmartindell/rmagick,tvon/rmagick,tvon/rmagick,rmagick/rmagick,antonifs/rmagick,rmagick/rmagick