Override plugins to build from environment
Override plugins to build from environment

This makes it easier to change the list of plugins to build:
```sh
PLUGINS=pygmentize python setup.py build
```

jay-z007/dxr,KiemVM/Mozilla--dxr,pelmers/dxr,gartung/dxr,nrc/dxr,bozzmob/dxr,pombredanne/dxr,nrc/dxr,kleintom/dxr,kleintom/dxr,erikrose/dxr,erikrose/dxr,pombredanne/dxr,bozzmob/dxr,jay-z007/dxr,jay-z007/dxr,bozzmob/dxr,srenatus/dxr,srenatus/dxr,gartung/dxr,nrc/dxr,KiemVM/Mozilla--dxr,jbradberry/dxr,kleintom/dxr,jbradberry/dxr,kleintom/dxr,bozzmob/dxr,gartung/dxr,gartung/dxr,pelmers/dxr,erikrose/dxr,KiemVM/Mozilla--dxr,bozzmob/dxr,jbradberry/dxr,srenatus/dxr,bozzmob/dxr,kleintom/dxr,nrc/dxr,nrc/dxr,pelmers/dxr,pelmers/dxr,jbradberry/dxr,jay-z007/dxr,jay-z007/dxr,KiemVM/Mozilla--dxr,KiemVM/Mozilla--dxr,pombredanne/dxr,srenatus/dxr,bozzmob/dxr,KiemVM/Mozilla--dxr,pelmers/dxr,jbradberry/dxr,gartung/dxr,pombredanne/dxr,pombredanne/dxr,jbradberry/dxr,jay-z007/dxr,pelmers/dxr,kleintom/dxr,erikrose/dxr,pombredanne/dxr,kleintom/dxr,erikrose/dxr,gartung/dxr,srenatus/dxr,pelmers/dxr,gartung/dxr,srenatus/dxr,jay-z007/dxr,pombredanne/dxr,jbradberry/dxr,nrc/dxr