gnu: ceph: Install headers to the "lib" output.

The previous provision for this has been defunct since version 12.

* gnu/packages/storage.scm (ceph)[arguments]: Remove obsolete substitution in
favor of a configure flag.

gnu/packages/storage.scm

@@ -93,6 +93,8 @@
               (libdir (string-append lib "/lib")))
          (list (string-append "-DCMAKE_INSTALL_PREFIX=" out)
                (string-append "-DCMAKE_INSTALL_LIBDIR=" libdir)
+               (string-append "-DCMAKE_INSTALL_INCLUDEDIR="
+                              lib "/include")
                ;; We need both libdir and libdir/ceph in RUNPATH.
                (string-append "-DCMAKE_INSTALL_RPATH="
                               libdir ";" libdir "/ceph")
@@ -127,11 +129,6 @@
              (let ((out (assoc-ref outputs "out"))
                    (lib (assoc-ref outputs "lib")))
 
-               ;; Make header files follow the dynamic libraries.
-               (substitute* "src/include/CMakeLists.txt"
-                 (("DESTINATION include")
-                  (string-append "DESTINATION " lib "/include")))
-
                (substitute* "cmake/modules/Distutils.cmake"
                  ;; Prevent creation of Python eggs.
                  (("setup.py install")