README: Distinguish source tarball vs git deps

autoconf/automake/libtool are only needed if building from git, as the dist
tarball is not depending on autotools.

The reference to GNU Autotools has been moved under "Building from Git", since
this detail is interesting only for those who clone the project.

Author: dacav

README

@@ -52,28 +52,25 @@ brew install pam-u2f
 [[building]]
 == Building from a source tarball
 
-This project uses 'autoconf', 'automake', 'pkg-config' and 'libtool'
-to achieve portability and ease of use.
+This project depends on https://developers.yubico.com/libfido2[`libfido2`]
+(>= 1.3.0).
+Versions of this project up to 1.0.8 used `libu2f-host` and
+`libu2f-server`.
 
-In addition, https://developers.yubico.com/libfido2['libfido2'] (>=
-1.3.0) is needed. Versions of this project up to 1.0.8 used
-`libu2f-host` and `libu2f-server`. On Ubuntu, the necessary dependencies can be
-installed using
+On Ubuntu, the dependencies needed to build the source tarball can be installed
+using:
 
 [source, console]
 ----
 sudo apt install --no-install-recommends \
-    autoconf \
-    automake \
     libfido2-dev \
     libpam-dev \
     libssl-dev \
-    libtool \
     pkg-config \
 ;
 ----
 
-If you downloaded a tarball, build it as follows.
+The project can be build as follows:
 
 [source, console]
 ----
@@ -104,9 +101,11 @@ This will create a directory 'pam-u2f'. Enter the directory:
 cd pam-u2f
 ----
 
-`autoconf`, `automake`, `libtool`, and `libpam` must be installed.
-`AsciiDoc` and `xsltproc` are used to generate the manpages. On Ubuntu,
-the necessary dependencies can be installed using
+This project uses `autoconf`, `automake`, `pkg-config` and `libtool`
+to achieve portability and ease of use.
+`AsciiDoc` and `xsltproc` are used to generate the manpages.
+
+On Ubuntu, the dependencies needed to build from Git can be installed using:
 
 [source, console]
 ----
@@ -125,7 +124,7 @@ sudo apt install --no-install-recommends
 ;
 ----
 
-On Fedora, the necessary dependencies can be installed using
+On Fedora, the necessary dependencies can be installed using:
 
 [source, console]
 ----