cmake: Add build-time dependency to Boost 1.56

We'll be using some Boost features in the future, so prepare the
build system to reflect that dependency.

CMakeLists.txt

@@ -321,6 +321,13 @@ if (BUILD_CORE)
 endif()
 
 # Non-Qt-based packages
+#####################################################################
+
+find_package(Boost 1.56 REQUIRED)
+set_package_properties(Boost PROPERTIES TYPE REQUIRED
+    URL "https://www.boost.org/"
+    DESCRIPTION "Boost libraries for C++"
+)
 
 find_package(ZLIB REQUIRED)
 set_package_properties(ZLIB PROPERTIES TYPE REQUIRED

INSTALL

@@ -33,11 +33,10 @@ compiler is needed:
 
 Other compilers may work, but are not officially supported.
 
-Furthermore, CMake 3.5 or later is required.
-
 As Quassel is a Qt application, you need the Qt SDK, version 5.5 or higher.
-
-There are several optional dependencies; we will talk about that later.
+Furthermore, the Boost header-only libraries (at least version 1.56) and
+CMake 3.5 or later are required. CMake will tell you about any missing
+dependencies when configuring the project.
 
 Compiling Quassel - short version
 ---------------------------------