Spent some of my morning fixing a build issue that came down to the use of AC_ARG_ENABLE. I thought it was worth recording some notes for its use:

Something enabled by default:

AC_ARG_ENABLE([badgers],
              [AC_HELP_STRING([--enable-badgers=@<:@yes/no@:>@],
                              [Enable badgers @<:@default=yes@:>@])],
              [],
              [enable_badgers=yes])

Something disabled by default:

AC_ARG_ENABLE([badgers],
              [AC_HELP_STRING([--enable-badgers=@<:@yes/no@:>@],
                              [Enable badgers @<:@default=no@:>@])],
              [],
              [enable_badgers=no])

Observe that in this case the only thing that changes is the default value in the third parameter to AC_ARG_ENABLE (and also the documentation :-))

Something enabled by default but with a –disable syntax:

AC_ARG_ENABLE([badgers],
              [AC_HELP_STRING([--disable-badgers],
                              [Disable badgers],
              [],
              [enable_badgers=yes])

Notice that this case is just the same as the first except with the help changed. Of course you actually need to use the value from the flag. I think this is more readable if presented outside the AC_ARG_ENABLE parameters this is possible because AC_ARG_ENABLE always sets a variable called enable_<thing>. Awesome, huh?

For conditional pkg-config:

AS_IF([test "x$enable_badgers" = "xyes"],
      [PKG_CHECK_MODULES(BADGERS, [badgers-1.0])],
      [])

For conditional building:

AM_CONDITIONAL(ENABLE_BADGERS, test "x$badgers" = xyes)