Fix CUPS_VERSION_MAJOR typo and add CUPS_CFLAGS for libcups3 builds

[rkt0209]

Summary

Three small, self-contained build fixes needed to compile cleanly against
libcups3 (CUPS 3.x), where the CUPS headers live off the default include
path and are only found via CUPS_CFLAGS. None of these change runtime
behaviour; they only correct the build.

Changes

1. cupsfilters/testfilters.c — fix CUPS_VERISON_MAJOR typo
   The macro name was misspelled, so the preprocessor treated it as 0.
   That made #if CUPS_VERISON_MAJOR < 3 always true, forcing the libcups2
   compatibility #defines onto a libcups3 build and breaking it. Corrected
   to CUPS_VERSION_MAJOR.

2. Makefile.am — add $(CUPS_CFLAGS) to the test programs
   testcmyk, testdither, testimage, testrgb, testpdf1, testpdf2,
   test-analyze, test-pdf and test-ps include libcupsfilters headers
   that pull in <cups/cups.h>. Without $(CUPS_CFLAGS) they fail to find
   the CUPS headers under libcups3 (cups/cups.h: No such file or directory).
   test1284 and testfilters already passed $(CUPS_CFLAGS); this brings
   the rest in line.

3. libcupsfilters.pc.in — add @CUPS_CFLAGS@ to Cflags
   The installed public headers #include <cups/cups.h>, so a downstream
   consumer compiling against pkg-config --cflags libcupsfilters also needs
   the CUPS include path. Without it, consumer builds break against libcups3.