Fix a failure to find "guild-VERSION" binary by falling back to
unversioned "guild".

Suggestion from https://notabug.org/guile-sqlite3/guile-sqlite3/issues/6

Index: guile-json-1.2.0/configure.ac
===================================================================
--- guile-json-1.2.0.orig/configure.ac
+++ guile-json-1.2.0/configure.ac
@@ -32,6 +32,11 @@ m4_pattern_forbid([^GUILE_PKG])
 
 dnl Check for Guile 2.x, and substitute 'GUILE_EFFECTIVE_VERSION'.
 GUILE_PKG([2.2 2.0])
+AC_ARG_VAR([GUILD], [guild (Guile compiler) command])
+AS_IF([test "x$GUILD" = "x"],
+  [PKG_CHECK_VAR([GUILD], [guile-$GUILE_EFFECTIVE_VERSION], [guild], [],
+    [AC_MSG_ERROR(m4_normalize([
+      'guild' binary not found; please check your Guile installation.]))])])
 GUILE_PROGS
 
 AC_CONFIG_FILES([Makefile json/Makefile])
