Convert assert_equal to stdarg for formats

It is nice to be able to override the numeric formatting when
using the cl_assert_equal helpers (in particular, I wanted to be
able to have octal formatting for numbers).  To allow this, I
consolidated the different clar__assert_equal_X helpers into a
single one that uses varargs and examines a new "format" input
that tells what format is expected for the two values to be
compared.

To test this, I expanded the sample tests to include broader
coverage and I made sure that each example test included both one
or more passing tests and a failing test.

Additionally, when the snprintf_eq macro was fixed to use variadic
arguments, the name no longer made sense (since it was no longer
being used just to print equality of two values).  I renamed the
whole macro to the more generic p_snprintf to indicate that it is
just a simple "portable" wrapper to snprintf.
Check before overwriting existing define

This should generate less warnings in some embedded environments
Fix a Win32 x64 warning
Merge pull request #40 from arrbee/more-assert-equal-fmts

More flexibility with cl_assert_equal_...