gnu: utf8-h: Install header under include/utf8 prefix.

This is to standardize with the usage of the library in the package's only
current user, OpenBoardView.  It also matches the installation layout of the
Arch package, which is made by its author.

* gnu/packages/c.scm (utf8-h) [arguments]: Use gexps.  Adjust installation
prefix to 'include/utf8' in install phase.
* gnu/packages/electronics.scm (openboardview)
[arguments]: Delete fix-utf8-include-directive phase.

gnu/packages/c.scm

@@ -1365,8 +1365,9 @@ (define-public utf8-h
                   "0x9f7ivww8c7cigf4ck0hfx2bm79qgx6q4ccwzqbzkrmcrl9shfb"))))
       (build-system cmake-build-system)
       (arguments
-       `(#:phases
-         (modify-phases %standard-phases
+       (list
+        #:phases
+        #~(modify-phases %standard-phases
             (delete 'build)
             (delete 'configure)
             (replace 'check
@@ -1377,8 +1378,8 @@ (define-public utf8-h
                     (invoke "make")))))
             (replace 'install
               (lambda* (#:key outputs #:allow-other-keys)
-               (let ((out (assoc-ref outputs "out")))
-                 (install-file "utf8.h" (string-append out "/include"))))))))
+                (install-file "utf8.h"
+                              (string-append #$output "/include/utf8")))))))
       (home-page "https://github.com/sheredom/utf8.h")
       (synopsis "Single header UTF-8 string functions for C and C++")
       (description "A simple one header solution to supporting UTF-8 strings in

gnu/packages/electronics.scm

@@ -298,14 +298,6 @@ (define keep (list "." ".." "openboardview"))
                   "add_subdirectory("
                   (search-input-directory inputs "share/glad") ;source_dir
                   " src/glad)\n")))))                          ;binary dir
-          (add-before 'configure 'fix-utf8-include-directive
-            ;; Our utf8-h package makes the header available as "utf8.h"
-            ;; directly rather than "utf8/utf8.h".
-            (lambda _
-              (substitute* '("src/openboardview/FileFormats/BRDFile.cpp"
-                             "src/openboardview/FileFormats/BRDFileBase.cpp"
-                             "src/openboardview/BoardView.cpp")
-                (("utf8/utf8.h") "utf8.h"))))
           (add-before 'configure 'dynamically-load-gtk-via-absolute-path
             ;; The GTK library is not linked thus not present in the RUNPATH of
             ;; the produced binary; the absolute path of the libraries must to