msvc: Check for _MSC_VER

MSVC requires special treatment. We check for it now.

Author: jdemel

include/volk/volk_complex.h

@@ -21,7 +21,7 @@
 #include <volk/volk_common.h>
 
 __VOLK_DECL_BEGIN
-#ifndef __STDC_NO_COMPLEX__
+#ifndef _MSC_VER
 // Obviously, we would love `typedef float complex lv_32fc_t` to work.
 // However, this clashes with C++ definitions.
 // error: expected initializer before ‘lv_32fc_t’