First, fix typo: "{$makeinfo_old}" should be "${makeinfo_old}". That
was preventing us from signaling an error.
Second, make sure if --disable-build-docs is set, we don't try to test
for the makeinfo version. Do this by moving all of the code inside
the test for enable_build_docs being true.
Added `makeinfo_old` variable that is set to true if makeinfo is too
old. Update that appropriately, and print out an error or message
based on the value of `makeinfo_old`.
Minor tweak to math.m4 so remove extra blank lines in the output
After each m4 command, add m4_dnl so that no extra blank lines are
inserted in the output. Perhaps not really necessary, but it makes
the output look more natural without the extra blank lines.