buildroot

e2fsprogs: fix build failure of subst.c

subst.c is built for the host, but uses defines calculated by the
configure script using the cross-compiler. By passing BUILD_CFLAGS, we
help subst.c include the proper header files, and therefore avoid the
build failure.

Fixes:

  http://autobuild.buildroot.org/results/2a66280800b8aa16fee2701b3f345aa24bb13c35/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>