[CMake] Generate cmake target that other libraries can use (#86)

Generates cmake target that other libraries can use to express
their dependency on this library and retrieve any configuration
information such as the include directory.

test/test_cmake/CMakeLists.txt

@@ -0,0 +1,21 @@
+# Copyright 2018 Mike Dev
+# Distributed under the Boost Software License, Version 1.0.
+# See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt
+#
+# NOTE: This does NOT run the unit tests for Boost.Predef.
+#       It only tests, if the CMakeLists.txt file in predef's
+#       root directory works as expected (i.e. it provides the
+#       target Boost::predef which in turn provides the
+#       correct include directory )
+
+cmake_minimum_required( VERSION 3.5 )
+
+project( BoostPredefCMakeSelfTest )
+
+# Process cmake file at root of library and use ${CMAKE_CURRENT_BINARY_DIR}/libs/boost_predef as workspace
+add_subdirectory( ../.. ${CMAKE_CURRENT_BINARY_DIR}/libs/boost_predef )
+
+add_executable( boost_predef_cmake_test_prj main.cpp )
+
+target_link_libraries( boost_predef_cmake_test_prj Boost::predef )
+

test/test_cmake/main.cpp

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+// Dummy executable, just to make sure
+// that we can find the Boost.predef header files
+#include <boost/predef.h>
+
+int main() {
+
+}