Fix compilation issue with gcc v6.x and empty CXXFLAGS

[chros73]

Fix compilation issue with gcc v6.x and empty CXXFLAGS:

• it removes -O2 flag and compilation can fail (e.g. with make -j4)

Note: CFLAGS are also effected.

Closes: #155
Refers to: rakshasa/rtorrent#628

Edit:
I have updated the fix:

• let the building environment (autotools) define what flags are used only if CFLAGS/CXXFLAGS is empty

[coveralls]

Coverage decreased (-0.02%) to 18.02% when pulling c2b119d on chros73:fix_cxxflags_check into c167c5a on rakshasa:master.

[coveralls]

Coverage increased (+0.003%) to 18.043% when pulling 8229218 on chros73:fix_cxxflags_check into c167c5a on rakshasa:master.