fix bash_completion install

Index: cmake/BashCompletion.cmake
--- cmake/BashCompletion.cmake.orig
+++ cmake/BashCompletion.cmake
@@ -71,9 +71,9 @@ ELSE()
 		SET(BASHCOMP_SCRIPT "${CMAKE_CURRENT_BINARY_DIR}/install_${SCRIPT_NAME}_completion.sh")
 
 		FILE(WRITE ${BASHCOMP_SCRIPT} "\
-#!${BASH}\n\
+#!/bin/sh\n\
 set -e\n\
-BASHCOMP_PKG_PATH=\"${BASHCOMP_USER_PATH}\"\n\
+BASHCOMP_PKG_PATH=\"\$DESTDIR${BASHCOMP_USER_PATH}\"\n\
 if [ -n \"${BASHCOMP_PKG_PATH}\" ]; then\n\
   BASHCOMP_PKG_PATH=\"${BASHCOMP_PKG_PATH}\"\n\
 fi\n\
