build: remove obsolete reference to bundled libiptc

Libiptc was removed 3 years ago in 99364c70, so this option to
./configure isn't of any use anymore.
build system: correct warning about 'INCLUDES'

This suppresses this warning when running build.sh:

Makefile.am:5: warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')
build system: correct warning about subdir-objects

This suppresses this category of warnings when running build.sh:

src/Makefile.am:NNN: warning: source file 'XXX' is in a subdirectory,
src/Makefile.am:NNN: but option 'subdir-objects' is disabled
Merge pull request #688 from mfournier/misc-build-cleanups

Misc build cleanups