can't run ppm_to_pixels on MacOSX

While building on MacOSX:
```
dyld: Library not loaded: @rpath/libhdf5.10.2.1.dylib
  Referenced from: /Users/vfonov/src/build/minc-toolkit-v2/Display/./ppm_to_pixels
  Reason: image not found
/bin/sh: line 1: 57626 Abort trap: 6           ./ppm_to_pixels /Users/vfonov/src/minc-toolkit-v2/Display/tools/checkbox.ppm /Users/vfonov/src/build/minc-toolkit-v2/Display/checkbox.h
```