Detect incompatible libtorrent v2 versions in build systems

Although the detection is working, the error message in autotools is not
perfect, i.e. it would only mention valid range for libtorrent v1 and it
shouldn't be a problem since v2 support isn't fully ready yet.

INSTALL

@@ -5,8 +5,8 @@ qBittorrent - A BitTorrent client in C++ / Qt
 
   - Boost >= 1.65
 
-  - libtorrent-rasterbar >= 1.2.14 (by Arvid Norberg)
-      * https://www.libtorrent.org/
+  - libtorrent-rasterbar: (>= 1.2.14) || (>= 2.0.4)
+      * By Arvid Norberg, https://www.libtorrent.org/
       * Be careful: another library (the one used by rTorrent) uses a similar name
 
   - OpenSSL >= 1.1.1
@@ -15,10 +15,12 @@ qBittorrent - A BitTorrent client in C++ / Qt
 
   - zlib >= 1.2.11
 
-  - pkg-config (compile-time only on *nix systems)
+  - pkg-config
+      * Compile-time only on *nix systems
 
-  - Python >= 3.5.0 (optional, runtime only)
-      * Required by the internal search engine
+  - Python >= 3.5.0
+      * Optional, run-time only
+      * Used by the bundled search engine
 
 Dependency version numbers are bumped every once in a while to keep the range of properly tested configurations manageable, even if not strictly required to build.
 You may be able to build with older versions of (some of) the dependencies other than the minimum versions specified in the build scripts, but support for such builds is not provided - you are on your own.