Specify own compilation flags in addition to one specified by environment

Different build package systems (on Gentoo or Arch) tend to specify a few
default compile/link flags. Makefile should add its own flags to the list.

Signed-off-by: Anatol Pomozov <anatol.pomozov@gmail.com>
Merge pull request #4 from anatol/master

Specify own compilation flags in addition to one specified by environment