build: improve make man

This improves `make man` in the following ways:

  * The recently added `osbuild-manifest.5` man-page is now generated
    as well.

  * The target now honors `SRCDIR` and `BUILDDIR` variables.

  * Any newly added man-page is now automatically picked up and
    generated as well.

  * The output directory structure now mirrors the input directory
    structure.

Makefile

@@ -17,6 +17,8 @@
 BUILDDIR ?= .
 SRCDIR ?= .
 
+RST2MAN ?= rst2man
+
 #
 # Automatic Variables
 #
@@ -88,9 +90,14 @@ $(BUILDDIR)/%/:
 # deployments to our website, as well as package manager scripts.
 #
 
+MANPAGES_RST = $(wildcard $(SRCDIR)/docs/*.[0123456789].rst)
+MANPAGES_TROFF = $(patsubst $(SRCDIR)/%.rst,$(BUILDDIR)/%,$(MANPAGES_RST))
+
+$(MANPAGES_TROFF): $(BUILDDIR)/docs/%: $(SRCDIR)/docs/%.rst | $(BUILDDIR)/docs/
+	$(RST2MAN) "$<" "$@"
+
 .PHONY: man
-man:
-	rst2man docs/osbuild.1.rst osbuild.1
+man: $(MANPAGES_TROFF)
 
 #
 # Building packages