fix #16949 --app:staticlib works on openbsd; --app:staticlib tested in CI (#16950)

compiler/extccomp.nim

@@ -124,8 +124,8 @@ compiler llvmGcc:
   result.name = "llvm_gcc"
   result.compilerExe = "llvm-gcc"
   result.cppCompiler = "llvm-g++"
-  when defined(macosx):
+  when defined(macosx) or defined(openbsd):
-    # OS X has no 'llvm-ar' tool:
+    # `llvm-ar` not available
     result.buildLib = "ar rcs $libfile $objfiles"
   else:
     result.buildLib = "llvm-ar rcs $libfile $objfiles"

tests/misc/tapp_lib_staticlib.nim

@@ -0,0 +1,27 @@
+discard """
+joinable: false
+"""
+
+# bug #16949
+
+when defined case1:
+  proc foo(): int {.exportc.} = 10
+elif defined case2:
+  proc foo(): int {.exportc, dynlib.} = 10
+elif defined caseMain:
+  proc foo(): int {.importc.}
+  doAssert foo() == 10
+else:
+  import stdtest/specialpaths
+  import std/[os, strformat, strutils, compilesettings]
+  proc runCmd(cmd: string) =
+    doAssert execShellCmd(cmd) == 0, $cmd
+  const
+    file = currentSourcePath
+    nim = getCurrentCompilerExe()
+    mode = querySetting(backend)
+  proc test(lib, options: string) =
+    runCmd fmt"{nim} {mode} -o:{lib} --nomain {options} -f {file}"
+    # runCmd fmt"{nim} r -b:{mode} --passl:{lib} -d:caseMain -f {file}" # pending https://github.com/nim-lang/Nim/pull/16945
+  test(buildDir / "libD20210205T172314.a", "--app:staticlib -d:nimLinkerWeakSymbols -d:case1")
+  test(buildDir / DynlibFormat % "D20210205T172720", "--app:lib -d:case2")