solver/brent.cpp: clang found a scoping bug

I introduced a bug when I ported brent over from scipy.
SConstruct: Minor changes for BlueGene

1. Use -std=c++11 instead of -std=c++0x.
2. Don't use -dead_strip with clang.
3. Make powerpc optimization flags the same as native's.
SConstruct: Make it easy to adjust optimization flags