Use StringValuePtr instead of STR2CSTR
Fix ruby 2.0 dl deprecation warning (use fiddle)
Properly set compiler defines in CPPFLAGS

CFLAGS are wrong, they won't be used for C++ source files. This fixes
the GEM not properly working for JPEG/PNG/TIFF on ruby 2.0.0.
Bump version to 0.2.6 (ruby 2.0 compatibility)