Use CheckForPthreads.cxx to find pthreads library on Ubuntu.

CMakeLists.txt

@@ -15,7 +15,7 @@ set (PACKAGE_STRING    "${PROJECT_NAME} ${PACKAGE_VERSION}")
 set (PACKAGE_TARNAME   "${PROJECT_NAME}-${PACKAGE_VERSION}")
 set (PACKAGE_BUGREPORT "https://code.google.com/p/gflags/issues/")
 
-project (${PROJECT_NAME} CXX C)
+project (${PROJECT_NAME} CXX)
 
 version_numbers (
   ${PACKAGE_VERSION}

cmake/CheckForPthreads.cxx

+#include <stdio.h>
+#include <pthread.h>
+#include <unistd.h>
+
+void* runner(void*);
+
+int res = 0;
+#ifdef __CLASSIC_C__
+int main(){
+  int ac;
+  char*av[];
+#else
+int main(int ac, char*av[]){
+#endif
+  pthread_t tid[2];
+  pthread_create(&tid[0], 0, runner, (void*)1);
+  pthread_create(&tid[1], 0, runner, (void*)2);
+
+#if defined(__BEOS__) && !defined(__ZETA__) // (no usleep on BeOS 5.)
+  usleep(1); // for strange behavior on single-processor sun
+#endif
+
+  pthread_join(tid[0], 0);
+  pthread_join(tid[1], 0);
+  if(ac > 1000){return *av[0];}
+  return res;
+}
+
+void* runner(void* args)
+{
+  int cc;
+  for ( cc = 0; cc < 10; cc ++ )
+    {
+    printf("%d CC: %d\n", (int)args, cc);
+    }
+  res ++;
+  return 0;
+}

cmake/FindThreadsCxx.cmake

@@ -100,7 +100,7 @@ else()
         message(STATUS "Check if compiler accepts -pthread")
         try_run(THREADS_PTHREAD_ARG THREADS_HAVE_PTHREAD_ARG
           ${CMAKE_BINARY_DIR}
-          ${CMAKE_ROOT}/Modules/CheckForPthreads.c
+          ${CMAKE_ROOT}/Modules/CheckForPthreads.cxx
           CMAKE_FLAGS -DLINK_LIBRARIES:STRING=-pthread
           COMPILE_OUTPUT_VARIABLE OUTPUT)