dune-debian.mk: Restore variable for configure options

While we currently don't have any module-specific options, it might be
useful to keep the option to set them available.

debian/dune-debian.mk

@@ -2,10 +2,12 @@ include /usr/share/dpkg/pkg-info.mk
 
 DUNE_CTEST ?= /usr/bin/dune-ctest
 
+DUNE_DEBIAN_CMAKE_FLAGS = -DBUILD_SHARED_LIBS=1
+
 DUNE_DEBIAN_SHLIB = $(subst ~,.,lib$(DEB_SOURCE)-$(DEB_VERSION_UPSTREAM))
 
 override_dh_auto_configure:
-	dh_auto_configure -- -DBUILD_SHARED_LIBS=1
+	dh_auto_configure -- $(DUNE_DEBIAN_CMAKE_FLAGS)
 
 # OpenMPI looks for a rsh program, even when it only starts local processes.
 export OMPI_MCA_plm_rsh_agent=/bin/false