if (...) then; AC_PROG_CC else AC_PROG_CC fi doesn't work?

[Brooks Moses]

Consider the following nearly-minimal configure.ac example:

=================================
AC_INIT
AC_ARG_ENABLE(foo)

if test "$enable_foo"; then
  echo "enabled"
  AC_PROG_CC([gcc])
else
  echo "disabled"
  AC_PROG_CC
fi
=================================

This does not work correctly; when --enable-foo is not given, the check 
for the C compiler will produce erroneous results (such as detecting GCC 
and then "checking whether we are using the GNU C compiler... no"), and 
some investigation shows that the problem is that ac_objext is "".

Further, inspection of the generated "configure" file shows the reason: 
The check for the object extension is only placed in the code for the 
first AC_PROG_CC, and thus gets omitted from the second one.

This is happening both with autoconf 2.59 and 2.61.  I haven't checked 
other versions.

Is this a known bug?  Is there a suggested workaround?

Thanks,
- Brooks