flex: propagate m4 dependency on wchar

Commit fe6a9e5e9d4046173a60235f8fd9a3b4746929c6 missed the fact that
m4 has a dependency on wide char support, so this dependency should be
propagated to flex.

Fixes:

  http://autobuild.buildroot.org/results/495/49502b2e33a346b2fcebf5e2da00af2661b54d6e/build-end.log

[Peter: Note where wchar dep comes from, show comment if not available]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/flex/Config.in

@@ -14,5 +14,9 @@ config BR2_PACKAGE_FLEX_BINARY
 	depends on BR2_PACKAGE_FLEX
 	# runtime dependency
 	select BR2_PACKAGE_M4
+	depends on BR2_USE_WCHAR # m4
 	help
 	  Install the flex binary tool in the target filesystem.
+
+comment "flex binary requires a toolchain with WCHAR support"
+	depends on !BR2_USE_WCHAR