CMakeLists.txt: support AVX2 detection

[luadebug]

CMakeLists.txt: support AVX2 detection for xmake-io/xmake-repo#8277
vcpkg produced somewhat similiar:
enable-avx2-on-gcc.patch

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 18c1b45..fe8d702 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -93,7 +93,7 @@ if (OMATH_STATIC_MSVC_RUNTIME_LIBRARY)
     )
 endif ()
 
-if (OMATH_USE_AVX2 AND CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
+if (OMATH_USE_AVX2 AND (CMAKE_CXX_COMPILER_ID STREQUAL "Clang" OR CMAKE_CXX_COMPILER_ID STREQUAL "GNU"))
     target_compile_options(${PROJECT_NAME} PUBLIC -mavx2 -mavx -mfma)
 endif ()

[orange-cpp]

granted, thx <3