gnu: alsa-modular-synth: Insert space to separate linker flags.

* gnu/packages/audio.scm (alsa-modular-synth)[arguments]: Add build
phase to separate linker flags.

gnu/packages/audio.scm

@@ -81,7 +81,16 @@
                 "1azbrhpfk4nnybr7kgmc7w6al6xnzppg853vas8gmkh185kk11l0"))))
     (build-system gnu-build-system)
     (arguments
-     `(#:configure-flags '("--enable-qt5")))
+     `(#:configure-flags '("--enable-qt5")
+       #:phases
+       (modify-phases %standard-phases
+         ;; Insert an extra space between linker flags.
+         (add-before 'configure 'add-missing-space
+           (lambda _
+             (substitute* "configure"
+               (("LIBS\\+=\\$LIBSsave") "LIBS+=\" $LIBSsave\"")
+               (("CFLAGS\\+=\\$CFLAGSsave") "CFLAGS+=\" $CFLAGSsave\""))
+             #t)))))
     (inputs
      `(("alsa-lib" ,alsa-lib)
        ;; We cannot use zita-alsa-pcmi (the successor of clalsadrv) due to