makefile-env: separate cppflags and cflags usage

AM_COMMON_FLAGS included both CPP and C[XX] flags. This can cause
re-definition errors like this:

make[4]: Nothing to be done for `all-am'.
CXX      cls/version/cls_version_client.o
<command-line>: warning: "__STDC_FORMAT_MACROS" redefined

because the macro definition appears in both AM_CXXFLAGS and
AM_CPPFLAGS.

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
automake: add per-target AM_CPPFLAGS

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
Merge pull request #616 from ceph/automake-flags

automake: fixup am_common_flags to avoid re-definitions

Reviewed-by: Roald J. van Loon <roaldvanloon@gmail.com>