Improve error message for `include_directories(abs_path_intree)`

This error message was quite confusing when triggered by
use of an absolute path to the include dir of an external dependency
(numpy in my case). Changing that to a relative dir also isn't
a solution, because Meson will *not* do the "busywork to make paths
work" that the error message says it will.

mesonbuild/interpreter/interpreter.py

@@ -2583,8 +2583,9 @@ class Interpreter(InterpreterBase, HoldableObject):
         for a in incdir_strings:
             if a.startswith(src_root):
                 raise InvalidArguments(textwrap.dedent('''\
-                    Tried to form an absolute path to a source dir.
-                    You should not do that but use relative paths instead.
+                    Tried to form an absolute path to a dir in the source tree.
+                    You should not do that but use relative paths instead, for
+                    directories that are part of your project.
 
                     To get include path to any directory relative to the current dir do
 
@@ -2599,6 +2600,10 @@ class Interpreter(InterpreterBase, HoldableObject):
                     put in the include directories by default so you only need to do
                     include_directories('.') if you intend to use the result in a
                     different subdirectory.
+
+                    Note that this error message can also be triggered by
+                    external dependencies being installed within your source
+                    tree - it's not recommended to do this.
                     '''))
             else:
                 try:

test cases/failing/44 abspath to srcdir/test.json

@@ -1,7 +1,7 @@
 {
   "stdout": [
     {
-      "line": "test cases/failing/44 abspath to srcdir/meson.build:3:0: ERROR: Tried to form an absolute path to a source dir."
+      "line": "test cases/failing/44 abspath to srcdir/meson.build:3:0: ERROR: Tried to form an absolute path to a dir in the source tree."
     }
   ]
 }