Add DEFAULT_TARGETS to base variable environment for Holmake

Allows for much more concise presentation of standard pattern when
additional targets are required, namely:

all: $(DEFAULT_TARGETS) mytarget1 mytarget2
.PHONY: all

Fix up many examples of previous verbose style.

Thanks to Magnus for the suggestion.

Closes #618

Implement rational calculation routine RAT_MUL_CONV

With tests.

More operations to follow of course.

Fix yet another Holmakefile problem.

New src/rational/Holmakefile records extra dependency for intExtensionTheory.
This dependency is genuine but can't be calculated by Holmake.