um: Allow static linking for non-glibc implementations

It is possible to produce a statically linked UML binary with UML_NET_VECTOR,
UML_NET_VDE and UML_NET_PCAP options enabled using alternative libc
implementations, which do not rely on NSS, such as musl.

Allow static linking in this case.

Signed-off-by: Ignat Korchagin <ignat@cloudflare.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Tested-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/Kconfig

@@ -62,12 +62,12 @@ config NR_CPUS
 
 source "arch/$(HEADER_ARCH)/um/Kconfig"
 
-config FORBID_STATIC_LINK
-	bool
+config MAY_HAVE_RUNTIME_DEPS
+        bool
 
 config STATIC_LINK
 	bool "Force a static link"
-	depends on !FORBID_STATIC_LINK
+	depends on CC_CAN_LINK_STATIC_NO_RUNTIME_DEPS || !MAY_HAVE_RUNTIME_DEPS
 	help
 	  This option gives you the ability to force a static link of UML.
 	  Normally, UML is linked as a shared binary.  This is inconvenient for

arch/um/drivers/Kconfig

@@ -234,7 +234,7 @@ config UML_NET_DAEMON
 config UML_NET_VECTOR
 	bool "Vector I/O high performance network devices"
 	depends on UML_NET
-	select FORBID_STATIC_LINK
+	select MAY_HAVE_RUNTIME_DEPS
 	help
 	This User-Mode Linux network driver uses multi-message send
 	and receive functions. The host running the UML guest must have
@@ -246,7 +246,7 @@ config UML_NET_VECTOR
 config UML_NET_VDE
 	bool "VDE transport (obsolete)"
 	depends on UML_NET
-	select FORBID_STATIC_LINK
+	select MAY_HAVE_RUNTIME_DEPS
 	help
 	This User-Mode Linux network transport allows one or more running
 	UMLs on a single host to communicate with each other and also
@@ -294,7 +294,7 @@ config UML_NET_MCAST
 config UML_NET_PCAP
 	bool "pcap transport (obsolete)"
 	depends on UML_NET
-	select FORBID_STATIC_LINK
+	select MAY_HAVE_RUNTIME_DEPS
 	help
 	The pcap transport makes a pcap packet stream on the host look
 	like an ethernet device inside UML.  This is useful for making