build: don't duplicate substitution of make variables

build: don't duplicate substitution of make variables

Thanks to our 'GIT_CONF_SUBST' layer in configure.ac, a make variable 'VAR'
can be defined to a value 'VAL' at ./configure runtime in our build system
simply by using "GIT_CONF_SUBST([VAR], [VAL])" in configure.ac, rather than
having both to call "AC_SUBST([VAR], [VAL])" in configure.ac and adding the
'VAR = @VAR@' definition in config.mak.in.  Less duplication, less margin
for error, less possibility of confusion.

While at it, fix some formatting issues in configure.ac that unnecessarily
obscured the code flow.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>