configure: explicit dependency on lowdown library

This dependency is used from quite a long time (now in libcmd) but
was not explicitly stated in the configure phase, possibly leading
to quite late build failures if that was not met (ie. building it
outside the .nix files provided). This MR adds it in the configure
phase so the failure is early and error is much more explicit.

configure.ac

@@ -260,6 +260,8 @@ AC_ARG_ENABLE(doc-gen, AS_HELP_STRING([--disable-doc-gen],[disable documentation
   doc_generate=$enableval, doc_generate=yes)
 AC_SUBST(doc_generate)
 
+# Look for lowdown library.
+PKG_CHECK_MODULES([LOWDOWN], [lowdown >= 0.8.0], [CXXFLAGS="$LOWDOWN_CFLAGS $CXXFLAGS"])
 
 # Setuid installations.
 AC_CHECK_FUNCS([setresuid setreuid lchown])