Makefile: don't export O to the environment

Several projects use the kernel style O=<dir> syntax to build out of tree,
and atleast uClibc doesn't check that it was explictly passed on the command
line, so setting it in the environment breaks the build.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
cairo: script backend needs png

Fixes http://autobuild.buildroot.org/results/7b21c60d7ba1a8df8910d104e21d4545cb37a4c6

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>