Makefile,meson.build: Enable/assume -Wextra

Enable all -Wextra warnings but -Wunused-parameter. Nobody seems to
miss warnings about unused parameters and we have a lot unavoidable
occurrences in flashrom because of common interfaces.

Change-Id: Id2ece264c2d483e34019985dd3a7631c4889abe6
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/30411
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jacob Garber <jgarber1@ualberta.ca>
diff --git a/Makefile b/Makefile
index b0c242d..f0a1b61 100644
--- a/Makefile
+++ b/Makefile
@@ -30,7 +30,7 @@
 DIFF    = diff
 PREFIX  ?= /usr/local
 MANDIR  ?= $(PREFIX)/share/man
-CFLAGS  ?= -Os -Wall -Wshadow -Wmissing-prototypes -Wwrite-strings
+CFLAGS  ?= -Os -Wall -Wextra -Wno-unused-parameter -Wshadow -Wmissing-prototypes -Wwrite-strings
 EXPORTDIR ?= .
 RANLIB  ?= ranlib
 PKG_CONFIG ?= pkg-config