Allow ./configure --with-oniguruma=builtin

This allows one to force the use of jq's builtin copy of Oniguruma
(builtin as a git submodule).

We still need to fix make dist to include that copy of Oniguruma.

configure.ac

@@ -236,11 +236,16 @@ AS_IF([test "x$with_oniguruma" != xno], [
    save_CFLAGS="$CFLAGS"
    save_LDFLAGS="$LDFLAGS"
    AS_IF([test "x$with_oniguruma" != xyes], [
+      AS_IF([test "x$with_oniguruma" = xbuiltin], [
+          build_oniguruma=yes
+      ], [
           onig_CFLAGS="-I${with_oniguruma}/include"
           onig_LDFLAGS="-L${with_oniguruma}/lib"
           CFLAGS="$CFLAGS $onig_CFLAGS"
           LDFLAGS="$LDFLAGS $onig_LDFLAGS"
       ])
+   ])
+   AS_IF([test "x$build_oniguruma" = xno], [
        # check for ONIGURUMA library
        AC_CHECK_HEADER("oniguruma.h",
           AC_CHECK_LIB([onig],[onig_version]))
@@ -249,11 +254,14 @@ AS_IF([test "x$with_oniguruma" != xno], [
 
        # handle check results
        AS_IF([test "x$ac_cv_lib_onig_onig_version" != "xyes"], [
+          build_oniguruma=yes
+          AC_MSG_NOTICE([Oniguruma was not found. Will use the packaged oniguruma.])
+       ])
+   ])
+   AS_IF([test "x$build_oniguruma" = xyes], [
        onig_CFLAGS="-I${srcdir}/modules/oniguruma/src"
        onig_LDFLAGS=
        AC_CONFIG_SUBDIRS([modules/oniguruma])
-      build_oniguruma=yes
-      AC_MSG_NOTICE([Oniguruma was not found. Will use the packaged oniguruma.])
    ])
    AC_SUBST(onig_CFLAGS)
    AC_SUBST(onig_LDFLAGS)