Darktable and RawSpeed upstream support two build modes: if BINARY_PACKAGE_BUILD==1
they pass -mtune=generic to the compiler, otherwise they use -march=native.
In either case, these options override externally set CFLAGS.

Disable this behaviour so that the users' settings provided to the ebuild by Portage
are respected.

--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -521,7 +521,7 @@
     set(DT_REQ_INSTRUCTIONS "-msse2")
   endif()
 
-  include(march-mtune)
+  set(MARCH "")
 
   #if(NOT BUILD_SSE2_CODEPATHS)
   #  set(MARCH "${MARCH} -mno-sse2 -D__DISABLE_SSE2__ -U__SSE2__ -D__DISABLE_SSE__ -U__SSE__")
--- a/src/external/rawspeed/cmake/compiler-flags.cmake
+++ b/src/external/rawspeed/cmake/compiler-flags.cmake
@@ -1,5 +1,4 @@
 include(CheckCXXCompilerFlag)
-include(CpuMarch)
 include(CheckCXXCompilerFlagAndEnableIt)
 
 # yes, need to keep both the CMAKE_CXX_FLAGS and CMAKE_CXX_STANDARD.