Add a test step to produce a minimal binary using libbpf.

This patch adds a test step to link a minimal program to libbpf library produced,
making sure that the library works.

travis-ci/managers/ubuntu.sh

@@ -20,4 +20,4 @@ if ! ldd build/libbpf.so | grep -q libelf; then
     exit 1
 fi
 make -j$((4*$(nproc))) -C src OBJDIR=../build DESTDIR=../install install
-rm -rf build install
+CFLAGS=${CFLAGS} $(dirname $0)/test_compile.sh