Be able to let findlib/dune to use libraries available from OPAM

Makefile

@@ -135,7 +135,7 @@ $(DOT_INSTALL_CHUNKS_FOR_OCAML): | ocaml/Makefile.config
 
 # CONFIGURATION FILES
 _build/solo5.conf: gen_solo5_conf.sh $(OCAML_IS_BUILT)
-	SYSROOT="$(MAKECONF_SYSROOT)" ./gen_solo5_conf.sh > $@
+	PREFIX="$(MAKECONF_PREFIX)" SYSROOT="$(MAKECONF_SYSROOT)" ./gen_solo5_conf.sh > $@
 
 _build/empty-META: | _build
 	touch $@

gen_solo5_conf.sh

@@ -8,9 +8,43 @@ checkopt() {
   fi
 }
 
+# [path(solo5)] is important because it explains, from the user's point of
+# view, where we could find the OCaml compiler to generate a Solo5 unikernel,
+# but also where we could find the OCaml libraries (which is what findlib is
+# all about).
+#
+# The current choice is to consider the artefacts available in an OPAM switch
+# as possible solutions to the dependencies of a unikernel. For example, if a
+# unikernel depends on the [fmt] library, we give the option of using
+# [fmt.cmx{,a}] from [opam install fmt]. Reusing what may have been compiled by
+# the host OCaml compiler and installed via OPAM poses no problem in principle,
+# since we are not changing the way an OCaml file is compiled between the host
+# OCaml compiler and our cross-compiler, which uses Solo5. In fact, the
+# [*.cmx{,a}] files generated by the two compilers are strictly the same.
+#
+# However, the difference will be in the generation of the object files and the
+# link, but the build-system can ensure that our compiler is used at these
+# times.
+#
+# The only problem concerns the [*.a] archives installed via OPAM. These are
+# not compiled with our C compiler for Solo5. The mirage tool takes care of
+# downloading the sources of the unikernel dependencies so that if they contain
+# C files, they can be compiled with our cross-compiler. However, this is a
+# choice relating to the mirage tool that we should not impose at this level
+# (that of simply offering a cross-compiler). We also know that with dune,
+# primacy is given to the sources available in the dune environment rather than
+# to the artefacts available via OPAM/findlib.
+#
+# So, compiling a project with our Solo5 toolchain gives the user the
+# possibility of reusing what is available in OPAM and already installed. On
+# the very specific question of [*.a] archives, it is up to the user to take
+# care not to reuse the [*.a] files offered by OPAM/findlib but to recompile
+# them themselves with our cross-compiler from the sources. In this case, the
+# mirage tool takes care of this detail, using opam monorepo.
+
 cat << EOF
-path(solo5) = "$SYSROOT/lib"
-destdir(solo5) = "$SYSROOT/lib"
+path(solo5) = "$SYSROOT/lib/:$PREFIX/lib"
+destdir(solo5) = "$PREFIX/lib"
 stdlib(solo5) = "$SYSROOT/lib/ocaml"
 ocamlopt(solo5) = "$SYSROOT/bin/ocamlopt$(checkopt ocamlopt)"
 ocamlc(solo5) = "$SYSROOT/bin/ocamlc$(checkopt ocamlc)"