Fix build-gnu-libstdc++.sh to not depend on toolchain's sysroot

CFLAGS and CXXFLAGS already contain correct --sysroot.  Add/export
CPPFLAGS to also contain --sysroot otherwise in NDK package
where toolchains/*/*/*/sysroot doesn't exist, libstdc++-v3/configure
may fail to find correct headers and fail the build.

Change-Id: Ia2810ec1820f581251532af3d31b9daa2f433bf3
1 file changed