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. Also removes the 'make
install' as it is not needed.

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

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