Fix double colon in path to CIncludes in Startup

[ryandesign]

Fixes failure to find files in CIncludes, e.g.:

### MWCPPC Usage Warning:
# can't locate include directory '/Users/rschmidt/mpw/3.5::Interfaces:CIncludes:'
### MWCPPC Compiler Error:
#	#include <ConditionalMacros.h>
#	                             ^
# the file 'ConditionalMacros.h' cannot be opened