configure: confvars: warn/error on bad key or value

$confvars is not supposed to hold the same key more than once, and
spaces in values won't be parsed correctly. Additionally, it's later
iterated using shell IFS-split - which will also apply globs.

We now abort if the value IFS-splits badly (spaces/glob/empty).

We now warn on duplicates, but still accept them like before.

Example duplicate: --enable-static --disable-static
Example spaces:    --config-foo="bar baz"
Example glob:      --config-foo=" * "

Note that globs currently already expand at  eval opt=\"$opt\"
before we get a chance to test it (but we still detect the spaces).
See commit message of 21272067 (boilerplate var=..) about removing it.

These tests are also performed on values which configure itself adds,
but currently there are no issues with those.

Also, default_conf() now uses confvars_has, which fixes the following:
- False-positive if the key is a substring of existing key or value.
- Incorrect test if the value contains '=' or sh pattern chars *?[] .
No-op, because current default_conf calls don't have such issues.

configure

@@ -70,8 +70,26 @@ assign_opt() {
   eval ${1#--}=\$2  # no risk of IFS/glob in [OPT]NAME
 }
 
+# succeed if $1 doesn't IFS-split funny (globs, spaces, ...)
+good_split() {
+  set -- "$1" $1
+  test $# = 2 && test "$1" = "$2"
+}
+
+# $1: NAME[=VALUE]  succeed if confvars has NAME or NAME=* element
+confvars_has() {
+  ! case " $confvars " in *" ${1%%=*} "* | *" ${1%%=*}="*)
+    false
+  esac
+}
+
+# [multiple] NAME or NAME=VAL
 confvars_set() {
-  confvars="$confvars $*"
+  for cv; do
+    good_split "$cv" || { echo "configure: ERROR: bad config '$cv'"; exit 1; }
+    confvars_has "$cv" && echo "configure: WARNING: duplicate config '$cv'"
+    confvars="$confvars $cv"
+  done
 }
 
 for opt do
@@ -221,9 +239,9 @@ default() # set variable unless already set and not empty
     test -n "$2" && eval : \${$1:=\$2}  # ': ${foo:=$2}'
 }
 
-default_conf() # add to confvars unless already present
+default_conf() # add one config to confvars unless already present
 {
-    test "${confvars%${1%=*}*}" = "${confvars}" && confvars_set "$1"
+    confvars_has "$1" || confvars_set "$1"
 }
 
 if test -z "${source_path#.}" ; then