Build uses low optimization option "-O" instead of the one provided from outside

[yurivict]

For example, I provided CXXFLAGS="-O2 -pipe -fstack-protector-strong -fno-strict-aliasing", but the build added -O which takes precedence:

c++ -O2 -pipe -fstack-protector-strong -fno-strict-aliasing   -W -O -DNDEBUG -I../build -c ../src/backward.cpp

[arminbiere]

I am unsure what the semantics of providing 'CXXFLAGS' actually should be (some of the compilation flags should still be set by configure). We could filter out those that 'configure' wants to set but are already in 'CXXFLAGS'. Is this what you want? In principle there is already a mechanism for those '-f' options, but the '-O' is unclear.

[yurivict]

The configure script shouldn't add any CXXFLAGS when the user provided CXXFLAGS flags.
Only when no original CXXFLAGS exists should configure add anything, except perhaps include directories.