gnu: gx-slow-gear-lv2: Fix SSE detection.

* gnu/packages/music.scm (gx-slow-gear-lv2)[arguments]: Add phase
"escape-shell-commands".

gnu/packages/music.scm

@@ -2307,6 +2307,17 @@ adjusts the amount of harmonics."))))
                  (base32
                   "0c6099h5qkv7ilsvxxcrzwy1h6lkld1srh3fvbjxyw9q34kbqsyl"))
                 (file-name (string-append name "-" version "-checkout"))))
+      (arguments
+       (substitute-keyword-arguments (package-arguments gx-guvnor-lv2)
+         ((#:phases phases)
+          `(modify-phases ,phases
+             (add-after 'unpack 'escape-shell-commands
+               (lambda _
+                 (substitute* "Makefile"
+                   (("cat ") "$(shell cat ")
+                   (("/dev/null") "/dev/null)")
+                   (("SSE_CFLAGS = \"\"") "SSE_CFLAGS ="))
+                 #t))))))
       (home-page "https://github.com/brummer10/GxSlowGear.lv2")
       (synopsis "Slow gear audio effect")
       (description "This package provides the LV2 plugin \"GxSlowGear\", a