Allow for overriding make tool from environment

gmake (GNU make) appears to handle some dependencies better here than
bmake (BSD make).



git-svn-id: svn://svn.savannah.nongnu.org/avrdude/trunk/avrdude@1515 81a1dc3b-b13d-400b-aceb-764788c761c2

Author: dl8dtl

tools/build-mingw32.sh

@@ -51,4 +51,4 @@ env \
     --host=$(../../config.guess) \
     --target=${tgt}
 
-make all
+${MAKE:-make} all