removed most gcc extensions in source - but still required

* replaced the c++ style '//' comments
* added explicit math defines M_PI, .. - if not already defined
* gcc function (inline) attributes still require the extensions :-(

Signed-off-by: hayati ayguen <h_ayguen@web.de>
diff --git a/test_pffft.cpp b/test_pffft.cpp
index f5d085d..a544870 100644
--- a/test_pffft.cpp
+++ b/test_pffft.cpp
@@ -41,6 +41,11 @@
 #include <string.h>
 #include <time.h>
 
+/* define own constants required to turn off g++ extensions .. */
+#ifndef M_PI
+  #define M_PI    3.14159265358979323846  /* pi */
+#endif
+
 /* maximum allowed phase error in degree */
 #define DEG_ERR_LIMIT 1E-4