Improved documentation.

Added entry to release notes.
Added note to documentation, regarding from which version on the
behavior is changed.

docs/markdown/Reference-manual.md

@@ -1575,8 +1575,8 @@ the following methods:
 
 - `get_define(definename)` returns the given preprocessor symbol's
   value as a string or empty string if it is not defined.
-  The method will concatenate string literals as the compiler would.
-  E.g. `"a" "b"` will become `"ab"`.
+  Starting with 0.47.0, this method will concatenate string literals as
+  the compiler would. E.g. `"a" "b"` will become `"ab"`.
 
 - `get_id()` returns a string identifying the compiler. For example,
   `gcc`, `msvc`, [and more](Reference-tables.html#compiler-ids).

docs/markdown/Release-notes-for-0.47.0.md

@@ -21,3 +21,9 @@ Added the function `subdir_done()`. Its invocation exits the current script at
 the point of invocation. All previously invoked build targets and commands are
 build/executed. All following ones are ignored. If the current script was
 invoked via `subdir()` the parent script continues normally.
+
+## Concatenate string literals returned from get_define
+
+After obtaining the value of a preprocessor symbol consecutive string literals
+are merged into a single string literal.
+For example a preprocessor symbol's value `"ab" "cd"` is returned as `"abcd"`.