configure: set SED to gsed (MacOS et al) or sed (GNU).

We use some GNU sed features and it's easier to require gsed than fix
them all (and keep noticing when they break).  MacOS comes with gsed
already!

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

configure

@@ -82,6 +82,24 @@ default_python()
     done
 }
 
+# We want GNU sed, which, among other things, recognizes '\t'.
+# We don't want to get confused with some random program called gsed,
+# so we actually test.
+default_sed()
+{
+    SED_BINS="gsed sed"
+    for s in $SED_BINS; do
+        if [ "$(which $s)" != "" ] ; then
+            if [ "$(printf x | $s 's/x/\t/')" = "	" ]; then
+		echo "$s"
+                return
+            fi
+        fi
+    done
+    echo "No valid sed found?" >&2
+    exit 1
+}
+
 # Takes PYTHON var
 default_pytest()
 {
@@ -184,6 +202,7 @@ set_defaults()
     fi
     echo CSANFLAGS = $CSANFLAGS
     PYTHON=${PYTHON-$(default_python)}
+    SED=${SED-$(default_sed)}
     PYTEST=${PYTEST-$(default_pytest $PYTHON)}
     COPTFLAGS=${COPTFLAGS-$(default_coptflags "$DEBUGBUILD")}
     CONFIGURATOR_CC=${CONFIGURATOR_CC-$CC}
@@ -212,6 +231,7 @@ usage()
     echo "    To override compile line for configurator itself"
     usage_with_default "PYTEST" "$PYTEST"
     usage_with_default "VALGRIND" "$VALGRIND"
+    usage_with_default "SED" "$SED"
 
     echo "Options include:"
     usage_with_default "--prefix=" "$PREFIX"
@@ -554,6 +574,7 @@ add_var SHA256SUM "$SHA256SUM"
 add_var FUZZING "$FUZZING"
 add_var RUST "$RUST"
 add_var PYTHON "$PYTHON"
+add_var SED "$SED"
 
 # Hack to avoid sha256 name clash with libwally: will be fixed when that
 # becomes a standalone shared lib.