Don't fail for on attempt to install scm (#22)

If ldoc is missing it results in error: Directory 'doc' not found.
Instead we should simply skip such cases and allow to install scm
version.

Closes #21

Author: olegrok

CMakeLists.txt

@@ -66,3 +66,10 @@ install(
   FILES ${CMAKE_CURRENT_SOURCE_DIR}/errors.lua
   DESTINATION ${TARANTOOL_INSTALL_LUADIR}
 )
+
+if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/doc)
+    install(
+        DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/doc
+        DESTINATION ${TARANTOOL_INSTALL_LUADIR}/${PROJECT_NAME}
+    )
+endif()

errors-scm-1.rockspec

@@ -20,6 +20,5 @@ build = {
     variables = {
         TARANTOOL_INSTALL_LUADIR = '$(LUADIR)',
     },
-    copy_directories = {'doc'},
 }