[nggb] Don't mess with user overrides to configuration.
[nggb] Don't mess with user overrides to configuration.

So-called precious variables (AC_ARG_VAR) are provided so that the user
can override what the configure script would otherwise do, so do not
modify their values if the user sets them. This should be a general
rule. User input into the configuration _process_ should be through
AC_ARG_WITH and AC_ARG_ENABLE.

git-svn-id: 03a8048b5ee8463be5048a3801110fb50f378627@12922 d0ab736e-dc22-4aeb-8dc9-08def0aa14fd

kmdouglass/Micro-Manager,kmdouglass/Micro-Manager