libiconv: add an error when both libiconv and locale are enabled

This error should never show up if all Buildroot dependencies are
correct. However, rather than failing horribly later on, catch this
particular case early on and error out.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/libiconv/libiconv.mk

@@ -23,3 +23,10 @@ ifneq ($(BR2_ENABLE_DEBUG),y)
 	$(STRIPCMD) $(STRIP_STRIP_UNNEEDED) $(TARGET_DIR)/usr/lib/libcharset.so.*
 endif
 	touch $@
+
+# Configurations where the toolchain supports locales and the libiconv
+# package is enabled are incorrect, because the toolchain already
+# provides libiconv functionality, and having both confuses packages.
+ifeq ($(BR2_PACKAGE_LIBICONV)$(BR2_ENABLE_LOCALE),yy)
+$(error Libiconv should never be enabled when the toolchain supports locales. Report this failure to Buildroot developers)
+endif