Prevent binaries in src from colliding with libc++ headers

Prevent binaries in src from colliding with libc++ headers

Discovered with Nix and LLVM 17. Headers inside of libc++ can easily
collide with binaries being linked in src. This results in clang trying
to include a binary as a header.

Fix this by removing '-I.' and '-I$(srcdir)' from AM_CPPFLAGS and
DEFAULT_INCLUDES in src/Makefile.am.

To facilitate this config/eu.am has been refactored.  New file
config/eu-common.am contains all of the old eu.am but with the
AM_CPPFLAGS definition removed. eu.am now includes eu-common.am and
contains the old AM_CPPFLAGS definition.

eu.am functionality does not change, but src/Makefile.am can instead
include eu-common.am and define its own AM_CPPFLAGS without causing a
"multiply defined" warning during autoreconf.

Signed-off-by: Tristan Ross <tristan.ross@midstall.com>