configure.ac: label all Automake conditionals

Automake allows optional labeling of all "if", "else", and "endif"
statements.  If you label them, Automake will check them for
correctness.  Very handy for if/else/endif statements that are deeply
nested and/or a long distance from each other.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>
Merge pull request #218 from jsquyres/pr/label-automake-conditionals

configure.ac: label all Automake conditionals