LLVM: work around bug in dragonfly bsd llvm-config for shared libs

Of course there are OS specific bugs for llvm-config as well, so work
around those too.

mesonbuild/dependencies/dev.py

@@ -173,6 +173,16 @@ class LLVMDependency(ConfigToolDependency):
 
     def _set_new_link_args(self):
         """How to set linker args for LLVM versions >= 3.9"""
+        if (mesonlib.is_dragonflybsd() and not self.static and
+                version_compare(self.version, '>= 4.0')):
+            # llvm-config on DragonFly BSD for versions 4.0, 5.0, and 6.0 have
+            # an error when generating arguments for shared mode linking, even
+            # though libLLVM.so is installed, because for some reason the tool
+            # expects to find a .so for each static library. This works around
+            # that.
+            self.link_args = self.get_config_value(['--ldflags'], 'link_args')
+            self.link_args.append('-lLLVM')
+            return
         link_args = ['--link-static', '--system-libs'] if self.static else ['--link-shared']
         self.link_args = self.get_config_value(
             ['--libs', '--ldflags'] + link_args + list(self.required_modules),