Minor cmake nitpicks (#969)

It's much nicer and less error prone to just use add_subdirectory to establish the include directory.

Hide the GNUInstallDirs module by placing it in the helper module. The intent being that the main CMakeLists.txt should have a little code as possible. So that readers can quickly understand the project.

Use include_guard() when available in cmake 3.10+

Co-authored-by: Juan Ramos <juanr0911@gmail.com>

include/CMakeLists.txt

@@ -0,0 +1,20 @@
+
+# Add include folders to the library and targets that consume it
+# the SYSTEM keyword suppresses warnings for users of the library
+#
+# By adding this directory as an include directory the user gets a
+# namespace effect.
+#
+# IE:
+#   #include <gsl/gsl>
+if(GSL_STANDALONE_PROJECT)
+    target_include_directories(GSL INTERFACE
+        $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>
+        $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
+    )
+else()
+    target_include_directories(GSL SYSTEM INTERFACE
+        $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>
+        $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
+    )
+endif()