ustr: not available for static-only configurations

ustr now unconditionally builds a shared library, so we have to mark
it as not available for static-only builds. The only reverse
dependencies of ustr is libsemanage, which was anyway already not
available for pure static builds.

Fixes:

  http://autobuild.buildroot.org/results/734/734975f6ac041e8aece07205c0b8966fe51e7df1/

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

package/ustr/Config.in

@@ -1,6 +1,11 @@
 config BR2_PACKAGE_USTR
 	bool "ustr"
+	# unconditionally builds a shared library
+	depends on !BR2_STATIC_LIBS
 	help
 	  A small, safe string library.
 
 	  http://www.and.org/ustr/
+
+comment "ustr needs a toolchain w/ dynamic library"
+	depends on BR2_STATIC_LIBS