Makefile: replace -O6 with -O3

-O6 is not a legal option of gcc. Unofficially gcc considers it to be
equivalent of -O3. clang chalks on it, though. This commit sets the 
default optimization flag to be -O3, like gcc actually considered it.

Review: https://reviewboard.asterisk.org/r/3280/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@409308 f38db490-d61c-443f-a65b-d21fe96a405b