build: Respect CFLAGS and LDFLAGS passed to make

configure cannot expect that the user will not pass additional CFLAGS
and LDFLAGS at make time. Use OVS_CFLAGS and OVS_LDFLAGS instead to
collect compiler and linker flags and substitute in Makefile.am.

This allows for:
./configure --with-dpdk=[...]
make CFLAGS=-Wno-error=foo

Signed-off-by: Thomas Graf <tgraf@noironetworks.com>
travis: Fix DPDK build and treat bad-function-cast warning as non-error

A missing " prevented the DPDK build in the matrix from functioning.
Correctly build DPDK as a single, shared library and point the OVS
build to the corresponding build directory.

Due to incorrect casts in the DPDK headers, we have to disable
bad-function as being treated as a warning for now.

Signed-off-by: Thomas Graf <tgraf@noironetworks.com>
Co-authored-by: Daniele Di Proietto <ddiproietto@vmware.com>