Implement GetThreadCount on Linux.

gtest/gtest-all.cc

@@ -8258,6 +8258,7 @@ void FilePath::Normalize() {
 # include <sys/stat.h>
 #else
 # include <unistd.h>
+# include <dirent.h>
 #endif  // GTEST_OS_WINDOWS_MOBILE
 
 #if GTEST_OS_MAC
@@ -8333,6 +8334,22 @@ size_t GetThreadCount() {
   }
 }
 
+#elif GTEST_OS_LINUX
+
+// Returns the number of threads running in the process, or 0 to indicate that
+// we cannot detect it.
+size_t GetThreadCount() {
+  size_t thread_count = 0;
+  if (DIR *dir = opendir("/proc/self/task")) {
+    while (dirent *entry = readdir(dir)) {
+      if (entry->d_name[0] != '.')
+        ++thread_count;
+    }
+    closedir(dir);
+  }
+  return thread_count;
+}
+
 #else
 
 size_t GetThreadCount() {