Add a missing linker flag for clang on macOS (#642)

* [WIP] Add a missing linker flag for clang on macOS

* Use `-lc++abi`

* Add comment for `STATIC_FLAG`

* Only Clang requires `-lc++abi`

cmake/ConfigRelease.cmake

@@ -14,10 +14,6 @@ function(enable_ipo)
     endif ()
 endfunction()
 
-if (NOT APPLE)
-    set(STATIC_FLAG "-static ")
-endif ()
-
 if (CMAKE_CXX_COMPILER_ID STREQUAL "AppleClang")
     # ref: https://stackoverflow.com/a/3801032
     set(CMAKE_CXX_FLAGS_RELEASE "-O3 -flto -mtune=native")
@@ -31,22 +27,31 @@ elseif (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
     SET(CMAKE_CXX_ARCHIVE_FINISH true)
     enable_ipo()
 elseif (CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
+    if (APPLE)
+        # For nix, ref: https://github.com/NixOS/nixpkgs/issues/166205
+        set(STATIC_FLAG "-lc++abi")
+    else ()
+        set(STATIC_FLAG "-static")
+    endif ()
+
     set(CMAKE_CXX_FLAGS_RELEASE "-O3")
     if (${CMAKE_CXX_COMPILER} MATCHES "-[0-9]+$")
         string(REGEX REPLACE [[.*clang\+\+(-[0-9]+)$]] [[lld\1]] LINKER ${CMAKE_CXX_COMPILER})
-        set(STATIC_LINK_FLAG "${STATIC_FLAG}-fuse-ld=${LINKER}")
+        set(STATIC_LINK_FLAG "${STATIC_FLAG} -fuse-ld=${LINKER}")
     else ()
         find_program(lld_EXECUTABLE lld)
         if (lld_EXECUTABLE)
-            set(STATIC_LINK_FLAG "${STATIC_FLAG}-fuse-ld=lld")
+            set(STATIC_LINK_FLAG "${STATIC_FLAG} -fuse-ld=lld")
         else ()
             set(STATIC_LINK_FLAG "${STATIC_FLAG}") # use `ld`
         endif ()
     endif ()
     enable_ipo()
 else ()
     set(CMAKE_CXX_FLAGS_RELEASE "-O3")
-    set(STATIC_LINK_FLAG "${STATIC_FLAG}")
+    if (NOT APPLE)
+        set(STATIC_LINK_FLAG "-static")
+    endif ()
     enable_ipo()
 endif ()