Patch Vc to avoid warnings about macro redefinitions

See VcDevel/Vc#247 for more information.

cmake/modules/SearchInstalledSoftware.cmake

@@ -1124,9 +1124,14 @@ if(vc AND NOT Vc_FOUND)
   set(Vc_LIBNAME "${CMAKE_STATIC_LIBRARY_PREFIX}Vc${CMAKE_STATIC_LIBRARY_SUFFIX}")
   set(Vc_LIBRARY "${Vc_ROOTDIR}/lib/${Vc_LIBNAME}")
 
+  if(UNIX)
+    set(VC_PATCH_COMMAND patch -p1 < ${CMAKE_SOURCE_DIR}/cmake/patches/vc-bit-scan-forward.patch)
+  endif()
+
   ExternalProject_Add(VC
     URL     ${Vc_SRC_URI}
     URL_HASH SHA256=68e609a735326dc3625e98bd85258e1329fb2a26ce17f32c432723b750a4119f
+    PATCH_COMMAND ${VC_PATCH_COMMAND}
     BUILD_IN_SOURCE 0
     BUILD_BYPRODUCTS ${Vc_LIBRARY}
     LOG_DOWNLOAD 1 LOG_CONFIGURE 1 LOG_BUILD 1 LOG_INSTALL 1

cmake/patches/vc-bit-scan-forward.patch

@@ -0,0 +1,28 @@
+From 01015d26dd6a66b2cb39740c05bbb7d5eb0f10a3 Mon Sep 17 00:00:00 2001
+From: Guilherme Amadio <amadio@cern.ch>
+Date: Fri, 6 Mar 2020 10:46:02 +0100
+Subject: [PATCH] Avoid redefinition of _bit_scan_{forward,reverse} macros
+---
+ Vc/common/bitscanintrinsics.h | 7 ++-----
+ 1 file changed, 2 insertions(+), 5 deletions(-)
+
+diff --git a/Vc/common/bitscanintrinsics.h b/Vc/common/bitscanintrinsics.h
+index 62543563..ce419e34 100644
+--- a/Vc/common/bitscanintrinsics.h
++++ b/Vc/common/bitscanintrinsics.h
+@@ -29,11 +29,8 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ #define VC_COMMON_BITSCANINTRINSICS_H_
+
+ #if defined(Vc_GCC) || defined(Vc_CLANG) || defined(Vc_APPLECLANG)
+-#  if Vc_GCC >= 0x40500
+-     // GCC 4.5.0 introduced _bit_scan_forward / _bit_scan_reverse
+-#    include <x86intrin.h>
+-#  else
+-     // GCC <= 4.4 and clang have x86intrin.h, but not the required functions
++#include <x86intrin.h>
++#  ifndef _bit_scan_forward
+ #    define _bit_scan_forward(x) __builtin_ctz(x)
+ #include "macros.h"
+ static Vc_ALWAYS_INLINE Vc_CONST int _Vc_bit_scan_reverse_asm(unsigned int x) {
+-- 
+2.25.1