configure: clean whitespace with [:space:]

https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions

Fixes NDKr10e on Cygwin,
CC_IDENT for it is defined as

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

configure

@@ -477,7 +477,7 @@ sh_quote(){
 }
 
 cleanws(){
-    echo "$@" | sed 's/^ *//;s/  */ /g;s/ *$//;s/\\r//g'
+    echo "$@" | sed 's/^ *//;s/[[:space:]][[:space:]]*/ /g;s/ *$//'
 }
 
 filter(){