rebuild.sh: Fix Linux build.

The previous patch broke the Linux build. The main reason was
that the definition of GLIB_CFLAGS/GLIB_LIBS which were exported
when building the linux-x86 binaries, were later picked by the
linux-x86_64 build incorrectly.

The fix ensures that the environment variables are undefined.
Also save the environment to $BUILD_DIR/environment.txt to better
diagnose problems in the future.