bootloader: check if gcc supports -Wno-error=unused-but-set-variable

Check if gcc supports `-Wno-error=unused-but-set-variable` before
trying to use it. Fixes bootloader building with old versions of
gcc, such as 4.4.3.

bootloader/wscript

@@ -560,11 +560,14 @@ def configure(ctx):
                 '-Wall',
                 '-Werror',
                 '-Wno-error=unused-variable',
-                '-Wno-error=unused-but-set-variable',
                 '-Wno-error=unused-function',
             ]
         )
 
+        # -Wno-error=unused-but-set-variable is not available on old gcc versions (e.g., 4.4.3)
+        if ctx.check_cc(cflags='-Wno-error=unused-but-set-variable', execute=False, mandatory=False):
+            ctx.env.append_value('CFLAGS', ['-Wno-error=unused-but-set-variable'])
+
     # ** Defines, includes **
 
     if not ctx.env.DEST_OS == 'win32':

news/7592.bootloader.rst

@@ -0,0 +1,3 @@
+Fix bootloader building with old versions of ``gcc`` that do not
+support the ``-Wno-error=unused-but-set-variable`` compiler flag
+(e.g., ``gcc`` v4.4.3).