fix sdcv error 'GLIBCXX_3.4.15' not found
It seems that when environment variable `CXX` is defined the `AM_CXXFLAGS`
is ignored. This patch adds `-static-libstdc++` directly in `LDFLAGS` so
that sdcv need not to link libstdc++.so at run time.
Merge pull request #229 from chrox/master

fix sdcv error 'GLIBCXX_3.4.15' not found