2015-09-19: always use /usr/bin/env in hashbang for better portability

always use /usr/bin/env in hashbang for better portability (especially for guix)

diff --git a/Makefile.am b/Makefile.am
--- a/Makefile.am
+++ b/Makefile.am
@@ -21,13 +21,13 @@ input.in.intermediate: ${wisp_SOURCES}
 
 .PHONY: syntaxtests.sh
 syntaxtests.sh : wisp.scm tests/runtests-scheme-preprocessor.sh
-	echo '#!/bin/bash' > @abs_top_builddir@/$@
+	echo '#!/usr/bin/env bash' > @abs_top_builddir@/$@
 	echo @abs_top_srcdir@/tests/runtests-scheme-preprocessor.sh @abs_top_srcdir@ @abs_top_builddir@ >> @abs_top_builddir@/$@
 	chmod +x @abs_top_builddir@/$@
 
 .PHONY: syntaxtestsreader.sh
 syntaxtestsreader.sh : wisp.scm tests/runtests-scheme-reader.sh
-	echo '#!/bin/bash' > @abs_top_builddir@/$@
+	echo '#!/usr/bin/env bash' > @abs_top_builddir@/$@
 	echo @abs_top_srcdir@/tests/runtests-scheme-reader.sh @abs_top_srcdir@ @abs_top_builddir@ >> @abs_top_builddir@/$@
 	chmod +x @abs_top_builddir@/$@
 
diff --git a/bootstrap.sh b/bootstrap.sh
--- a/bootstrap.sh
+++ b/bootstrap.sh
@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Bootstrap wisp-guile with wisp.py
 if [[ x"$1" == x"" ]]; then 
diff --git a/wisp-multiline.sh b/wisp-multiline.sh
--- a/wisp-multiline.sh
+++ b/wisp-multiline.sh
@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # wisp-multiline.sh --- run multiline wisp code
 
 # Copyright (C) 2013 Arne Babenhauserheide <arne_bab@web.de>
diff --git a/wisp-repl-guile.sh b/wisp-repl-guile.sh
--- a/wisp-repl-guile.sh
+++ b/wisp-repl-guile.sh
@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/usr/bin/env bash
 
 # if the spec file does not exist yet, run the build chain
 if test ! -f language/wisp/spec.scm; then