build: fix ./configure options' descriptions

The description of

--enable/disable-experimental-features was a bogus "Developer mode, good
for testing" and

--enable/disable-valgrind was "Valgrind binary to use for tests" which
gave the false impression that it should be set to something like
/usr/local/bin/valgrind whereas it is a boolean option.

configure

@@ -139,11 +139,11 @@ usage()
     usage_with_default "--enable/disable-developer" "$DEVELOPER" "enable" "disable"
     echo "    Developer mode, good for testing"
     usage_with_default "--enable/disable-experimental-features" "$EXPERIMENTAL_FEATURES" "enable" "disable"
-    echo "    Developer mode, good for testing"
+    echo "    Enable experimental features"
     usage_with_default "--enable/disable-compat" "$COMPAT" "enable" "disable"
     echo "    Compatibility mode, good to disable to see if your software breaks"
     usage_with_default "--enable/disable-valgrind" "(autodetect)"
-    echo "    Valgrind binary to use for tests"
+    echo "    Run tests with Valgrind"
     usage_with_default "--enable/disable-static" "$STATIC" "enable" "disable"
     echo "    Static link sqlite3, gmp and zlib libraries"
     usage_with_default "--enable/disable-address-sanitizer" "$ASAN" "enable" "disable"