gnu: doxygen: Update to 1.8.16.

* gnu/packages/documentation.scm (doxygen): Update to 1.8.16.
[arguments]: Add phase 'remove-git-requirement'.
* gnu/packages/patches/doxygen-test.patch: Adjust for upstream changes.

gnu/packages/documentation.scm

@@ -125,7 +125,7 @@ markup) can be customized and extended by the user.")
 (define-public doxygen
   (package
     (name "doxygen")
-    (version "1.8.15")
+    (version "1.8.16")
     (home-page "http://www.doxygen.nl/")
     (source (origin
              (method url-fetch)
@@ -136,7 +136,7 @@ markup) can be customized and extended by the user.")
                                        ".src.tar.gz")))
              (sha256
               (base32
-               "0p94b4yb6bk2dxzs5kyl82xxgq2qakgbx5yy3ssbbadncb20x75x"))
+               "10iwv8bcz5b5cd85gg8pgn0bmyg04n9hs36xn7ggjjnvynv1z67z"))
              (patches (search-patches "doxygen-test.patch"))))
     (build-system cmake-build-system)
     (native-inputs
@@ -157,6 +157,13 @@ markup) can be customized and extended by the user.")
              '())
        #:test-target "tests"
        #:phases (modify-phases %standard-phases
+                  (add-after 'unpack 'remove-git-requirement
+                    (lambda _
+                      ;; TODO: Remove this for > 1.8.16.
+                      (substitute* "cmake/git_watcher.cmake"
+                        (("Git QUIET REQUIRED")
+                         "Git QUIET"))
+                      #t))
                   (add-before 'configure 'patch-sh
                               (lambda* (#:key inputs #:allow-other-keys)
                                 (substitute* "src/portable.cpp"