Tweak autotools compilation

Add -pipe to potentially speed up compiles
Change -mtune to generic to clarify intent
(people compiling executables for their own use (i.e. not for distribution)
should set CXXFLAGS="-march=native" before running autogen.sh)

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@9824 387891d4-d844-0410-90c0-e4c51a9137d3