The LANG vars aren't reset early enough so when sed tries to use [a-zA-Z] in 
option parsing, it may break.

http://bugs.gentoo.org/105369

--- xterm/configure
+++ xterm/configure
@@ -46,6 +46,16 @@
 infodir='${prefix}/info'
 mandir='${prefix}/man'
 
+# NLS nuisances.
+# Only set these to C if already set.  These must not be set unconditionally
+# because not all systems understand e.g. LANG=C (notably SCO).
+# Fixing LC_MESSAGES prevents Solaris sh from translating var values in `set'!
+# Non-C LC_CTYPE values break the ctype check.
+if test "${LANG+set}"   = set; then LANG=C;   export LANG;   fi
+if test "${LC_ALL+set}" = set; then LC_ALL=C; export LC_ALL; fi
+if test "${LC_MESSAGES+set}" = set; then LC_MESSAGES=C; export LC_MESSAGES; fi
+if test "${LC_CTYPE+set}"    = set; then LC_CTYPE=C;    export LC_CTYPE;    fi
+
 # Initialize some other variables.
 subdirs=
 MFLAGS= MAKEFLAGS=
@@ -502,16 +512,6 @@
   esac
 done
 
-# NLS nuisances.
-# Only set these to C if already set.  These must not be set unconditionally
-# because not all systems understand e.g. LANG=C (notably SCO).
-# Fixing LC_MESSAGES prevents Solaris sh from translating var values in `set'!
-# Non-C LC_CTYPE values break the ctype check.
-if test "${LANG+set}"   = set; then LANG=C;   export LANG;   fi
-if test "${LC_ALL+set}" = set; then LC_ALL=C; export LC_ALL; fi
-if test "${LC_MESSAGES+set}" = set; then LC_MESSAGES=C; export LC_MESSAGES; fi
-if test "${LC_CTYPE+set}"    = set; then LC_CTYPE=C;    export LC_CTYPE;    fi
-
 # confdefs.h avoids OS command line length limits that DEFS can exceed.
 rm -rf conftest* confdefs.h
 # AIX cpp loses on an empty file, so make sure it contains at least a newline.