audioreactive: Move flags to library

Use a PlatformIO script to move the last of AR_buildflags in to the
module itself.

usermods/audioreactive/library.json

@@ -3,8 +3,7 @@
   "build": {
     "srcDir": ".",
     "includeDir": "../../wled00",
-    "libLDFMode": "chain+",
-    "libArchive": false
+    "extraScript": "override_sqrt.py"
   },
   "dependencies": [
     {

usermods/audioreactive/override_sqrt.py

@@ -0,0 +1,5 @@
+Import('env')
+
+for lb in env.GetLibBuilders():
+    if lb.name == "arduinoFFT":
+        lb.env.Append(CPPDEFINES=[("sqrt_internal", "sqrtf")])