Support building C++ files with MSVC

Made appropriate changes to be able to successfully
build C++ files using a Visual C++ build on Windows.

Based on an earlier patch by Kyle Schwarz.

Comments:

-- compat/w32pthreads.h: Made appropriate changes to w32pthreads.h to
   get it to build when it is being included in a C++ file and built
   with Visual C++.  This is mostly a copy of Kyle Schwarz's patch as
   described above.

-- configure:
a) Now calling set_ccvars CXX to cause the various CXX_ variables to
   be setup properly.  For example, with MSVC (Microsoft Visual C++),
   this causes CXX_O to be set to -Fo$@ instead of using the default
   value.  The default value does not work with Visual C++.  This
   change will also have the impact of correcting CXX_O (and possibly
   CXX_C) for other compilers, although this is really only relevant
   for the Intel compiler, in addition to MSVC.
b) Now using cl for the C++ compiler for the MSVC toolchain.  This is
   currently only relevant for building the
   Blackmagic/Decklink-related files under avdevice.
Signed-off-by: Hendrik Leppkes <h.leppkes@gmail.com>

compat/w32pthreads.h

@@ -77,7 +77,7 @@ typedef struct pthread_cond_t {
 
 static av_unused unsigned __stdcall attribute_align_arg win32thread_worker(void *arg)
 {
-    pthread_t *h = arg;
+    pthread_t *h = (pthread_t*)arg;
     h->ret = h->func(h->arg);
     return 0;
 }
@@ -270,7 +270,7 @@ static av_unused int pthread_cond_init(pthread_cond_t *cond, const void *unused_
     }
 
     /* non native condition variables */
-    win32_cond = av_mallocz(sizeof(win32_cond_t));
+    win32_cond = (win32_cond_t*)av_mallocz(sizeof(win32_cond_t));
     if (!win32_cond)
         return ENOMEM;
     cond->Ptr = win32_cond;
@@ -288,7 +288,7 @@ static av_unused int pthread_cond_init(pthread_cond_t *cond, const void *unused_
 
 static av_unused int pthread_cond_destroy(pthread_cond_t *cond)
 {
-    win32_cond_t *win32_cond = cond->Ptr;
+    win32_cond_t *win32_cond = (win32_cond_t*)cond->Ptr;
     /* native condition variables do not destroy */
     if (cond_init)
         return 0;
@@ -305,7 +305,7 @@ static av_unused int pthread_cond_destroy(pthread_cond_t *cond)
 
 static av_unused int pthread_cond_broadcast(pthread_cond_t *cond)
 {
-    win32_cond_t *win32_cond = cond->Ptr;
+    win32_cond_t *win32_cond = (win32_cond_t*)cond->Ptr;
     int have_waiter;
 
     if (cond_broadcast) {
@@ -337,7 +337,7 @@ static av_unused int pthread_cond_broadcast(pthread_cond_t *cond)
 
 static av_unused int pthread_cond_wait(pthread_cond_t *cond, pthread_mutex_t *mutex)
 {
-    win32_cond_t *win32_cond = cond->Ptr;
+    win32_cond_t *win32_cond = (win32_cond_t*)cond->Ptr;
     int last_waiter;
     if (cond_wait) {
         cond_wait(cond, mutex, INFINITE);
@@ -369,7 +369,7 @@ static av_unused int pthread_cond_wait(pthread_cond_t *cond, pthread_mutex_t *mu
 
 static av_unused int pthread_cond_signal(pthread_cond_t *cond)
 {
-    win32_cond_t *win32_cond = cond->Ptr;
+    win32_cond_t *win32_cond = (win32_cond_t*)cond->Ptr;
     int have_waiter;
     if (cond_signal) {
         cond_signal(cond);
@@ -397,7 +397,7 @@ static av_unused int pthread_cond_signal(pthread_cond_t *cond)
 static av_unused void w32thread_init(void)
 {
 #if _WIN32_WINNT < 0x0600
-    HANDLE kernel_dll = GetModuleHandle(TEXT("kernel32.dll"));
+    HMODULE kernel_dll = GetModuleHandle(TEXT("kernel32.dll"));
     /* if one is available, then they should all be available */
     cond_init      = (void (WINAPI*)(pthread_cond_t *))
         GetProcAddress(kernel_dll, "InitializeConditionVariable");

configure

@@ -3640,8 +3640,10 @@ case "$toolchain" in
         cl_major_ver=$(cl 2>&1 | sed -n 's/.*Version \([[:digit:]]\{1,\}\)\..*/\1/p')
         if [ -z "$cl_major_ver" ] || [ $cl_major_ver -ge 18 ]; then
             cc_default="cl"
+            cxx_default="cl"
         else
             cc_default="c99wrap cl"
+            cxx_default="c99wrap cl"
         fi
         ld_default="$source_path/compat/windows/mslink"
         nm_default="dumpbin -symbols"
@@ -3852,6 +3854,7 @@ msvc_common_flags(){
             -lz)                  echo zlib.lib ;;
             -lavicap32)           echo vfw32.lib user32.lib ;;
             -lx264)               echo libx264.lib ;;
+            -lstdc++)             ;;
             -l*)                  echo ${flag#-l}.lib ;;
             -LARGEADDRESSAWARE)   echo $flag ;;
             -L*)                  echo -libpath:${flag#-L} ;;
@@ -4197,6 +4200,7 @@ cflags_noopt=$_cflags_noopt
 add_cflags $_flags $_cflags
 cc_ldflags=$_ldflags
 set_ccvars CC
+set_ccvars CXX
 
 probe_cc hostcc "$host_cc"
 host_cflags_filter=$_flags_filter