Fix build failure if CMAKE_CXX_FLAGS is not empty

The wrong " were adding a new parameter, leading to an expanded flag list like
"-g -O2 -fstack-protector --param=ssp-buffer-size=4 -Wformat -Werror=format-security -D_FORTIFY_SOURCE=2 ;-std=c++0x"
and an obvious
"g++-4.8.real: fatal error: no input files"
error message.