Renname DDOC_VARS_STABLE -> DDOC_VARS_STABLE_HTML

[wilzbach]

Follow-up to 4129e45

The term DDOC_VARS_STABLE doesn't relate to DDOC_VARS_HTML and DDOC_VARS_VERBATIM.
Following the existing naming schemas, it prolly should be named DDOC_VARS_HTML_STABLE,

OK (see this PR).

or maybe you just use 2 variables DDOC_VARS_HTML and DDOC_VARS_STABLE (for the compiler) here.

I am already proud that I reduced them from being spread everyone to one common place ;-)
Do I understand you correctly that instead of:

DDOC_VARS=\
	DMD="${DMD}" \
	DRUNTIME_PATH="${DRUNTIME_DIR}" \
	DOCSRC="$(PWD)" \
	VERSION="${DMD_DIR}/VERSION"

DDOC_VARS_HTML=$(DDOC_VARS) \
	DOC_OUTPUT_DIR="${DOC_OUTPUT_DIR}/phobos-prerelease" \
	STDDOC="$(addprefix $(PWD)/, $(STD_DDOC_PRE))"

DDOC_VARS_VERBATIM=$(DDOC_VARS) \
	DOC_OUTPUT_DIR="${DOC_OUTPUT_DIR}/phobos-prerelease-verbatim" \
	STDDOC="$(PWD)/verbatim.ddoc"

you would want sth. like?

DDOC_VARS_HTML=\
	DMD="${DMD}" \
	DRUNTIME_PATH="${DRUNTIME_DIR}" \
	DOCSRC="$(PWD)" \
	VERSION="${DMD_DIR}/VERSION"
	DOC_OUTPUT_DIR="${DOC_OUTPUT_DIR}/phobos-prerelease" \
	STDDOC="$(addprefix $(PWD)/, $(STD_DDOC_PRE))"

DDOC_VARS_VERBATIM=$(DDOC_VARS_HTML) \
	DOC_OUTPUT_DIR="${DOC_OUTPUT_DIR}/phobos-prerelease-verbatim" \
	STDDOC="$(PWD)/verbatim.ddoc"

I am not convinced yet that this is nicer...