manual: Make the manual more available

The following changes should make the manual more available, and easier
to understand:
  1) "make dist" and "make install" will now include all translations of
     the manual.
  1) A "freedoom-" prefix will be used in all cases in place of just
     "manual".
  2) All languages will have a two letter suffix, so English is now
     "-en".
  3) Only the PDF specified will be built when invoked in the top level
     directory. For example:
       make manual/freedoom-manual-en.pdf

manual/Makefile

@@ -1,14 +1,21 @@
+# Generate the Freedoom manual PDF files based on ADOC files. Each translation
+# has a two character code. Adding a new translation does not require any
+# makefile changes, but it would be nice to keep the following table up to date:
+#
+# Two Character Code    Language
+# ------------------    --------
+#                 en    English
+#                 es    Spanish
+#                 fr    French
+
 ASCIIDOCTOR_FLAGS = -a pdf-page-size=A5
-all: freedoom-manual.pdf freedoom-manual-es.pdf freedoom-manual-fr.pdf
+MANUAL_ADOC_FILES=$(wildcard freedoom-manual-??.adoc)
+MANUAL_PDF_FILES=$(subst .adoc,.pdf,$(MANUAL_ADOC_FILES))
 
-freedoom-manual.pdf: manual.adoc
-	-asciidoctor-pdf $(ASCIIDOCTOR_FLAGS) manual.adoc -o $@
+all: $(MANUAL_PDF_FILES)
 
-freedoom-manual-es.pdf: manual-es.adoc
-	-asciidoctor-pdf $(ASCIIDOCTOR_FLAGS) manual-es.adoc -o $@
-
-freedoom-manual-fr.pdf: manual-fr.adoc
-	-asciidoctor-pdf $(ASCIIDOCTOR_FLAGS) manual-fr.adoc -o $@
+freedoom-manual-%.pdf: freedoom-manual-%.adoc
+	-asciidoctor-pdf $(ASCIIDOCTOR_FLAGS) $< -o $@
 
 clean:
-	$(RM) freedoom-manual.pdf freedoom-manual-es.pdf freedoom-manual-fr.pdf
+	$(RM) $(MANUAL_PDF_FILES)