build: Respect CFLAGS and LDFLAGS passed to make

configure cannot expect that the user will not pass additional CFLAGS
and LDFLAGS at make time [0]. 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

[0] http://www.gnu.org/software/automake/manual/html_node/Flag-Variables-Ordering.html

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
so far. This patch enables the DPDK build by properly building DPDK
as a single library and by pointing the OVS build to the corresponding
build directory. Also removes the 'make install' as it is not required
and only slows down the build.

Due to incorrect casts in the DPDK headers, we have to disable
bad-function-cast and cast-align 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>