simplify absolute unix path detection

use the posix builtin that is used two lines down as well instead of sed
with platform-specific options.

Change-Id: I439f79554d883564150004c1f7b6d8655a0ef192
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>

config.tests/unix/makeabs

@@ -2,14 +2,7 @@
 
 FILE="$1"
 RES="$FILE"
-
-CUT_ARG="-b1"
-if [ `uname -s` = "QNX" ]; then
-    # QNX does not understand "-b1"
-    CUT_ARG="-c1"
-fi
-
-if [ `echo $FILE | cut $CUT_ARG` = "/" ]; then
+if [ -z "${FILE##/*}" ]; then
     true
 elif [ "$OSTYPE" = "msys" -a -z "${FILE##[a-zA-Z]:[/\\]*}" ]; then
     true