Clarify how to generate the configure script

Move instructions on how to generate the configure script before the
rest of the commands on how to install with the configure script.

Closes #891

INSTALL.adoc

@@ -41,7 +41,16 @@ Before upgrading, you are advised to read link:NEWS.{docext}[the release notes].
 Installation using `configure`
 ------------------------------
 
-Optionally, you can use the `configure` script to detect dependencies:
+Optionally, you can use the `configure` script to detect dependencies.  If you
+downloaded the Tig sources from https://github.com/jonas/tig/releases/[a
+release] there's already a `configure` script. However, if you are building from
+the Tig repository, you need to generate the `configure` script yourself. First,
+ensure that `autoconf` is installed on your system, and then run the following
+command:
+
+	$ make configure
+
+To install using `configure`, run the following commands:
 
 	$ ./configure
 	$ make
@@ -56,12 +65,6 @@ build without `configure`):
 
         $ ./configure LDFLAGS=-L/usr/local/opt/ncurses/lib CPPFLAGS=-I/usr/local/opt/ncurses/include
 
-Note, if you are building from the Tig repository, you need to generate
-`configure` yourself. First, ensure that `autoconf` is installed on your system,
-and then run the following command:
-
-	$ make configure
-
 Installation using Homebrew
 ---------------------------
 You can use link:http://brew.sh[Homebrew] to install Tig on OS X: