BUG: 'make clean' failed for wmake/src

- caused by the typo '@E' instead of '$E' (commit 997f1713)

ENH: minor improvements for wmake/src makefile

- 'make clean' now also tries to remove the parent platforms/
  directory if possible.

- the flex intermediate build target is placed into the platforms/
  directory to avoid touching the src/ directory at all.

- suppress warnings about unused functions (GCC only)