dpkg-buildflags: implement --export=configure

The goal is to make it easy to set compilation flags on a ./configure
command line for example like this:
./configure $(shell dpkg-buildflags --export=configure)

The output thus looks like this:
CFLAGS="-g -O2" CPPFLAGS="" CXXFLAGS="-g -O2" FFLAGS="-g -O2" LDFLAGS=""