[build] SCons build with brotli needs -lm on *BSD

SConstruct

@@ -505,7 +505,10 @@ if 1:
 	if env['with_brotli']:
 		if not autoconf.CheckParseConfigForLib('LIBBROTLI', 'pkg-config libbrotlienc --cflags --libs'):
 			fail("Couldn't find libbrotlienc")
-		autoconf.env.Append(CPPFLAGS = [ '-DHAVE_BROTLI_ENCODE_H', '-DHAVE_BROTLI' ])
+		autoconf.env.Append(
+                        CPPFLAGS = [ '-DHAVE_BROTLI_ENCODE_H', '-DHAVE_BROTLI' ],
+                        LIBS = [ 'm' ],
+                )
 
 	if env['with_dbi']:
 		if not autoconf.CheckLibWithHeader('dbi', 'dbi/dbi.h', 'C'):