Add bracket for sed 4.3 compliance

Based on upstream fix by Daniel Stenberg in 09 jan 2017
See: https://hg.mozilla.org/mozilla-central/rev/ebcbf47a83e7

--- a/js/src/configure	2017-02-21 16:56:42.350105741 -0300
+++ b/js/src/configure	2017-02-21 17:33:13.183493978 -0300
@@ -15231,7 +15231,7 @@
         fi
     fi
 
-    version=`sed -n 's/^[:space:]*#[[:space:]]*define[[:space:]][[:space:]]*U_ICU_VERSION_MAJOR_NUM[[:space:]][[:space:]]*\([0-9][0-9]*\)[[:space:]]*$/\1/p' "$icudir/common/unicode/uvernum.h"`
+    version=`sed -n 's/^[[:space:]]*#[[:space:]]*define[[:space:]][[:space:]]*U_ICU_VERSION_MAJOR_NUM[[:space:]][[:space:]]*\([0-9][0-9]*\)[[:space:]]*$/\1/p' "$icudir/common/unicode/uvernum.h"`
     if test x"$version" = x; then
        { echo "configure: error: cannot determine icu version number from uvernum.h header file $lineno" 1>&2; echo "configure: error: cannot determine icu version number from uvernum.h header file $lineno" 1>&5; exit 1; }
     fi
