distro: gcc: Patch to allow builds without /bin/sh.

* distro/packages/base.scm (gcc-4.7): In `pre-configure' phase, patch
  shebang in `gcc/exec-tool.in'.

distro/packages/base.scm

@@ -428,6 +428,9 @@ BFD (Binary File Descriptor) library, `gprof', `nm', `strip', etc.")
 ~a~%"
                             libc line))))
 
+               ;; Adjust hard-coded #!/bin/sh.
+               (patch-shebang "gcc/exec-tool.in")
+
                ;; Don't retain a dependency on the build-time sed.
                (substitute* "fixincludes/fixincl.x"
                  (("static char const sed_cmd_z\\[\\] =.*;")