Backport from https://github.com/Itseez/opencv/commit/1f983ec39c97298b0c8ce409a1cc229ecf14e55c

From 1f983ec39c97298b0c8ce409a1cc229ecf14e55c Mon Sep 17 00:00:00 2001
From: Maksim Shabunin <maksim.shabunin@itseez.com>
Date: Tue, 9 Jun 2015 13:59:48 +0300
Subject: [PATCH] Fixed compilation of pthread-based parallel_for with gcc
 4.4.3

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
---
 modules/core/src/parallel.cpp | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/modules/core/src/parallel.cpp b/modules/core/src/parallel.cpp
index b1e7567..0b593ee 100644
--- a/modules/core/src/parallel.cpp
+++ b/modules/core/src/parallel.cpp
@@ -132,8 +132,14 @@
 namespace cv
 {
     ParallelLoopBody::~ParallelLoopBody() {}
+#if defined HAVE_PTHREADS && HAVE_PTHREADS
+    void parallel_for_pthreads(const cv::Range& range, const cv::ParallelLoopBody& body, double nstripes);
+    size_t parallel_pthreads_get_threads_num();
+    void parallel_pthreads_set_threads_num(int num);
+#endif
 }
 
+
 namespace
 {
 #ifdef CV_PARALLEL_FRAMEWORK
@@ -301,7 +307,7 @@ void cv::parallel_for_(const cv::Range& range, const cv::ParallelLoopBody& body,
         }
 
 #elif defined HAVE_PTHREADS
-        void parallel_for_pthreads(const Range& range, const ParallelLoopBody& body, double nstripes);
+
         parallel_for_pthreads(range, body, nstripes);
 
 #else
@@ -361,8 +367,6 @@ int cv::getNumThreads(void)
 
 #elif defined HAVE_PTHREADS
 
-        size_t parallel_pthreads_get_threads_num();
-
         return parallel_pthreads_get_threads_num();
 
 #else
@@ -424,8 +428,6 @@ void cv::setNumThreads( int threads )
 
 #elif defined HAVE_PTHREADS
 
-    void parallel_pthreads_set_threads_num(int num);
-
     parallel_pthreads_set_threads_num(threads);
 
 #endif
-- 
2.4.4