avr-binutils @2.41: opportunistically uses isl

[ryandesign (Ryan Carsten Schmidt)]

:info:configure checking for isl 0.15 or later... no
:info:configure required isl version is 0.15 or later
...
:warn:configure The following existing files were hidden from the build system by trace mode:
...
:msg:configure   /opt/local/include/isl/schedule.h

% port provides /opt/local/include/isl/schedule.h
/opt/local/include/isl/schedule.h is provided by: isl
% port installed isl
The following ports are currently installed:
  isl @0.24_1 (active)