modules: explicitly link to pthread in FindVulkan.

Originally I thought this was caused by the changes that happened in the
previous commit, but apparently not, and it was due to an unrelated
system upgrade I did -- the 1.2.202 Arch package had libvulkan.so link
to libpthread.so, but the 1.2.203 build not (and I couldn't reproduce
when rebuilding the 1.2.203 package from scratch, so I guess there was
some weird hiccup on the machine that used to build the package?). And
to make things worse, this caused a problem only when passing
`--magnum-device cpu` to select SwiftShader, which (unlike other
drivers, apparently) relies on pthread being linked in.

Nevertheless, Magnum should be immune against such issues same as it's
trying to be immune against Assimp regressions, so let's just link to
pthread always in the Find module, and do it via -pthread instead of
-lpthread so the same issue with the reference DCEd doesn't happen to
us.

modules/FindVulkan.cmake

@@ -78,10 +78,25 @@ find_library(Vulkan_LIBRARY
 include(FindPackageHandleStandardArgs)
 find_package_handle_standard_args(Vulkan DEFAULT_MSG Vulkan_LIBRARY)
 
+# libvulkan.so *should* link against pthread on its own, but in some rare
+# corner cases (like the 1.2.203 ArchLinux package) it doesn't -- most probably
+# due to the vulkan loader wrongly using `-lpthread` instead of `-pthread` and
+# then the linker DCEing out the reference to it based on some unfortunate
+# combination of compiler/linker flags?
+#
+# In that case using drivers that rely on pthread (such as SwiftShader) would
+# lead to a nasty std::system_error telling me to link to pthreads. As I can't
+# have control over how the loader links to pthread or whether at all, and what
+# random packaging issues happened in various distributions, I have to link to
+# it explicitly to be immune against such issues.
+set(THREADS_PREFER_PTHREAD_FLAG TRUE)
+find_package(Threads REQUIRED)
+
 if(NOT TARGET Vulkan::Vulkan)
     add_library(Vulkan::Vulkan UNKNOWN IMPORTED)
-    set_property(TARGET Vulkan::Vulkan PROPERTY
-        IMPORTED_LOCATION ${Vulkan_LIBRARY})
+    set_target_properties(Vulkan::Vulkan PROPERTIES
+        IMPORTED_LOCATION ${Vulkan_LIBRARY}
+        INTERFACE_LINK_LIBRARIES Threads::Threads)
 endif()
 
 mark_as_advanced(Vulkan_LIBRARY)