stuck with prefork if you have out-of-tree APR

[Jeff Trawick]

server/mpm/config.m4 has this check with a hard-coded path to APR:

  APR_CHECK_APR_DEFINE(APR_HAS_THREADS, srclib/apr)

This fails if APR is anywhere else and config doesn't think APR has 
thread support.

Is this macro busted and should be replaced with something that does a 
test compile using the current $INCLUDES in $CPPFLAGS?

I suspect the macro isn't cool since it assumes that APR header files 
are in the include subdirectory of whatever was passed in for the APR 
path.  What it needs to get to is the apr-config program, after which it 
can query for what goes in CPPFLAGS/INCLUDES/etc.