Makefile: work around ksh's failure to handle missing list argument to for loop

Makefile: work around ksh's failure to handle missing list argument to for loop

ksh does not like it when the list argument is missing in a 'for' loop.
This can happen when NO_CURL is set which causes REMOTE_CURL_ALIASES to be
unset.  In this case, the 'for' loop in the Makefile is expanded to look
like this:

   for p in ; do

and ksh complains like this:

   /bin/ksh: syntax error at line 15 : `;' unexpected

The existing attempt to work around this issue, introduced by 70b89f87,
tried to protect the 'for' loop by first testing whether REMOTE_CURL_ALIASES
was empty, but this does not work since, as Johannes Sixt explains, "Before
the test for emptyness can happen, the complete statement must be parsed,
but ksh finds a syntax error in the statement and, therefore, cannot even
begin to execute the statement. (ksh doesn't follow POSIX in this regard,
where this would not be a syntax error.)".

Make's $(foreach) function could be used to avoid this shell glitch, but
since it has already caused a problem once before by generating a command
line that exceeded the maximum argument list length on IRIX, let's adopt
Bruce Stephens's suggestion for working around this issue in the same way
the OpenSSL folks have done it.  This solution first assigns the contents
of the REMOTE_CURL_ALIASES make variable to a shell variable and then
supplies the shell variable as the list argument in the 'for' loop.  This
satisfies ksh and has the expected behavior even if $(REMOTE_CURL_ALIASES)
is empty.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>