Improve variable parsing when generating MINFO

Before this change, variables for which a '=' appeared in the assignment would
be parsed as the entire string up until the final '='. For example:

  BUILD_CMD=shlib_target=; if [ -n "$(SHARED_LIBS)" ]; then \

would result in the variable name "BUILD_CMD=shlib_target". This doesn't
appear to harm the current generation of MINFO, but creates problems for other
Makefile-related work I'm attempting.

Change-Id: I1f3a606d67fd5464bb459e8f36c23b3e967b77e1
Signed-off-by: Mike Bland <mbland@acm.org>
Signed-off-by: Geoff Thorpe <geoff@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>