build: Drop Werror=variadic-macros

The following commit:
80db8fd86c ("build: Use -Wvariadic-macros warning")
added a warning about variadic-macros.  But it isn't quite clear why
since variadic macros are used throghout iwd.  GCC doesn't honor this
option, but clang does.  Since there's no real reason to stop using
variadic macros at this time, drop this warning.

acinclude.m4

@@ -58,7 +58,6 @@ AC_DEFUN([COMPILER_FLAGS], [
 		CFLAGS+=" -Wdeclaration-after-statement"
 		CFLAGS+=" -Wmissing-declarations"
 		CFLAGS+=" -Wredundant-decls"
-		CFALGS+=" -Wvariadic-macros"
 		CFLAGS+=" -Wformat -Wformat-security"
 		if ( $CC -v 2>/dev/null | grep "gcc version" ); then
 			CFLAGS+=" -Wcast-align"