2015-10-12: use GUILE_PROGS to check for guile instead of using homegrown stuff.

use GUILE_PROGS to check for guile instead of using homegrown stuff.

diff --git a/Makefile.am b/Makefile.am
--- a/Makefile.am
+++ b/Makefile.am
@@ -17,10 +17,10 @@ ChangeLog :
 
 .INTERMEDIATE: input.in.intermediate
 input.in.intermediate: ${wisp_SOURCES} wisp.scm
-	@abs_top_srcdir@/bootstrap-reader.sh @abs_top_srcdir@ @abs_top_builddir@ @guile@ wisp.scm 2>&1 | sed "s/^;;;.*//" 2>&1 | grep . 1>&2 ; test ! $$? -eq 0 # it worked if grep does not find anything
+	@abs_top_srcdir@/bootstrap-reader.sh @abs_top_srcdir@ @abs_top_builddir@ @GUILE@ wisp.scm 2>&1 | sed "s/^;;;.*//" 2>&1 | grep . 1>&2 ; test ! $$? -eq 0 # it worked if grep does not find anything
 
 wisp.scm: wisp-guile.w wisp.py
-	@abs_top_srcdir@/bootstrap.sh @abs_top_srcdir@ @guile@ @python3@ 2>&1 | sed "s/^;;;.*//" 2>&1 | grep . 1>&2 ; test ! $$? -eq 0 # it worked if grep does not find anything
+	@abs_top_srcdir@/bootstrap.sh @abs_top_srcdir@ @GUILE@ @python3@ 2>&1 | sed "s/^;;;.*//" 2>&1 | grep . 1>&2 ; test ! $$? -eq 0 # it worked if grep does not find anything
 
 .PHONY: syntaxtests.sh
 syntaxtests.sh : wisp.scm tests/runtests-scheme-preprocessor.sh
diff --git a/configure.ac b/configure.ac
--- a/configure.ac
+++ b/configure.ac
@@ -5,10 +5,12 @@ AC_INIT([wisp], [0.9.0],
         [arne_bab@web.de])
 # Check for programs I need for my build
 AC_CANONICAL_TARGET
-AC_ARG_VAR([guile], [How to call GNU Guile 2.0.x.])
-AC_CHECK_TARGET_TOOL([guile], [guile-2.0], [no])
-AS_IF([test "x$guile" = "xno"],
-      [AC_MSG_ERROR([cannot find GNU Guile 2.0 or later.])])
+
+# search for Guile using the guile m4 files.
+# see https://www.gnu.org/software/guile/manual/html_node/Autoconf-Macros.html
+# This provides @GUILE@ to Makefile.am
+GUILE_PROGS([2.0.11])
+
 AC_ARG_VAR([python3], [How to call Python 3.])
 AC_CHECK_TARGET_TOOL([python3], [python3], [no])
 AS_IF([test "x$python3" = "xno"],