cmake: Allow using Posix threads.

Unavoidable with newer MinGW versions.

CMakeLists.txt

@@ -66,24 +66,6 @@ add_compiler_flags (-fno-strict-aliasing)
 
 include (StaticCRT)
 
-# Avoid Posix threads.  Posix threads is required for support of certain C++11
-# multi-threading features, but it introduces a new DLL dependency and we don't
-# use those features.
-execute_process (
-    COMMAND "${CMAKE_COMMAND}" -E echo "#include <thread>\n#ifdef _GLIBCXX_HAS_GTHREADS\n#error _GLIBCXX_HAS_GTHREADS\n#endif"
-    COMMAND "${CMAKE_CXX_COMPILER}" -x c++ -E -
-    RESULT_VARIABLE STATUS_CXX11_THREADS
-    OUTPUT_QUIET
-    ERROR_QUIET
-)
-if (NOT STATUS_CXX11_THREADS EQUAL 0)
-    if (POSIX_THREADS)
-        message (WARNING "Win32 threads recommended.")
-    else ()
-        message (SEND_ERROR "Win32 threads required.")
-    endif ()
-endif ()
-
 # Enable stack protection
 # XXX: Broken on https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86832
 if (CMAKE_CXX_COMPILER_VERSION VERSION_LESS "8.0" OR

cmake/StaticCRT.cmake

@@ -8,15 +8,26 @@ if (MINGW)
         set (CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -static-libgcc")
         set (CMAKE_MODULE_LINKER_FLAGS "${CMAKE_MODULE_LINKER_FLAGS} -static-libgcc")
     endif ()
-    if (CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
-        set (CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -static")
-    else ()
+    if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
         check_cxx_compiler_flag (-static-libstdc++ HAVE_STATIC_LIBSTDCXX_FLAG)
         if (HAVE_STATIC_LIBSTDCXX_FLAG)
             set (CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -static-libstdc++")
             set (CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -static-libstdc++")
             set (CMAKE_MODULE_LINKER_FLAGS "${CMAKE_MODULE_LINKER_FLAGS} -static-libstdc++")
         endif ()
+
+        # Statically link Posix threads when detected.
+        execute_process (
+            COMMAND "${CMAKE_COMMAND}" -E echo "#include <thread>\n#ifdef _GLIBCXX_HAS_GTHREADS\n#error _GLIBCXX_HAS_GTHREADS\n#endif"
+            COMMAND "${CMAKE_CXX_COMPILER}" -x c++ -E -
+            RESULT_VARIABLE STATUS_CXX11_THREADS
+            OUTPUT_QUIET
+            ERROR_QUIET
+        )
+        if (NOT STATUS_CXX11_THREADS EQUAL 0)
+            # https://stackoverflow.com/a/28001271
+            set (CMAKE_CXX_STANDARD_LIBRARIES "-Wl,-Bstatic -lstdc++ -lpthread -Wl,-Bdynamic ${CMAKE_CXX_STANDARD_LIBRARIES}")
+        endif ()
     endif ()
 endif ()