build: Add the ability to use a different version of valac

Document the need to use --disable-unversioned

Author: rrthomas

Makefile

@@ -1,4 +1,4 @@
-VALAC = valac
+VALAC ?= valac
 PACKAGES ?= --all
 GENERATOR_OPTS ?= --disable-devhelp --skip-existing
 VALAC_VERSION := $(shell $(VALAC) --api-version | awk -F. '{ print "0."$$2 }')

README.md

@@ -52,6 +52,12 @@ you completed building, you should see a `valadoc.org` folder.
 To access the documentation navigate your browser to http://localhost:7777.
 
 
+Using a different version of Vala
+=================================
+
+You may want or need to test Valadoc with a different version of Vala, for example to pick up fixes in Vala that are necessary to build example code. You can do this with a chroot or jhbuild; or if your valac’s API version is different from other versions you have installed, just set `VALAC=valac-X.YY` in your environment when running `make`.
+
+
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