configure: parse --enable/--disable-strip automatically, flip default

Always include the STRIP variable in config-host.mak (it's only used
by the s390-ccw firmware build, and it adds a default if configure
omitted it), and use meson-buildoptions.sh to turn
--enable/--disable-strip into -Dstrip.

The default is now not to strip the binaries like for almost every other
package that has a configure script.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

scripts/meson-buildoptions.py

@@ -36,6 +36,10 @@ SKIP_OPTIONS = {
     "trace_file",
 }
 
+BUILTIN_OPTIONS = {
+    "strip",
+}
+
 LINE_WIDTH = 76
 
 
@@ -90,14 +94,17 @@ def allow_arg(opt):
     return not (set(opt["choices"]) <= {"auto", "disabled", "enabled"})
 
 
+def filter_options(json):
+    if ":" in json["name"]:
+        return False
+    if json["section"] == "user":
+        return json["name"] not in SKIP_OPTIONS
+    else:
+        return json["name"] in BUILTIN_OPTIONS
+
+
 def load_options(json):
-    json = [
-        x
-        for x in json
-        if x["section"] == "user"
-        and ":" not in x["name"]
-        and x["name"] not in SKIP_OPTIONS
-    ]
+    json = [x for x in json if filter_options(x)]
     return sorted(json, key=lambda x: x["name"])