Fixes for Windows build

- Replace ../../psopt-installer-master with actual location of patches in $PSOPT_BUILD_DIR
- Add PSOPT=../.. to final make install invocation, so Makefile_include.mk can be successfully located

- Use -lopenblas instead of -llibopenblas, ~~the former works on both MinGW and Cygwin while the latter only works on MinGW~~ (nevermind, can probably be left out, see line comment at L91)