In http://www.gnu.org/software/autoconf/manual/autoconf.html#Conditional-constructs , under the definition for m4_ifval, I believe the following clause is erroneous:
This is shorthand for: m4_if([cond], [], [if-true], [if-false]) Since the latter expands to if-true if cond is empty, while the definition for m4_ifval says m4_ifval([cond], [if-true], [if-false]) expands to if-true if cond is NOT empty. This was rather confusing and I had to verify the correct behaviour via a configure.ac template. Hope this is corrected. Thanks, Mostafa