Enable -avx2 -fma for ntru even when compiling without AVX2 support

This code is runtime guarded with CPU feature tests, so even if we are
making a non-AVX2 build we still want to compile the avx code path with
-mavx2 -mfma so that it can be opportunistically used when run on
AVX2-supporting CPUs (particularly for the debs and release binaries).

crypto/CMakeLists.txt

@@ -37,7 +37,6 @@ set(NTRU_REF_SRC
 )
 
 set(NTRU_SRC
-  ${NTRU_AVX_SRC}
   ${NTRU_REF_SRC}
   libntrup/src/ntru.cpp
   )
@@ -47,21 +46,41 @@ set(CRYPTOGRAPHY_SRC ${NTRU_SRC})
 add_library(${CRYPTOGRAPHY_LIB} STATIC ${CRYPTOGRAPHY_SRC})
 add_log_tag(${CRYPTOGRAPHY_LIB})
 
+# The avx implementation uses runtime CPU feature detection to enable itself, so we *always* want to
+# compile it with avx2 support even if we aren't compiling with AVX2 enabled.
+add_library(cryptography_avx_lib STATIC ${NTRU_AVX_SRC})
+if(USE_AVX2)
+  # Assume cxxflags are already enabling AVX2
+else()
+  include(CheckCXXCompilerFlag)
+  check_cxx_compiler_flag(-mavx2 COMPILER_SUPPORTS_AVX2)
+  check_cxx_compiler_flag(-mfma COMPILER_SUPPORTS_FMA)
+  if(COMPILER_SUPPORTS_AVX2 AND COMPILER_SUPPORTS_FMA)
+    target_compile_options(cryptography_avx_lib PRIVATE -mavx2 -mfma)
+    message(STATUS "Building libntrup with runtime AVX2/FMA support")
+  else()
+    message(STATUS "Not building with libntrup runtime AVX2/FMA support (can't figure out how to compile with AVX2/FMA: -mavx2 -mfma didn't work)")
+  endif()
+endif()
+target_link_libraries(${CRYPTOGRAPHY_LIB} PRIVATE cryptography_avx_lib)
+
 option(DOWNLOAD_SODIUM "Allow libsodium to be downloaded and built locally if not found on the system" OFF)
 
 find_package(Sodium 1.0.17)
 
 if(sodium_FOUND)
   target_include_directories(${CRYPTOGRAPHY_LIB} PUBLIC ${sodium_INCLUDE_DIR})
-  target_link_libraries(${CRYPTOGRAPHY_LIB} ${sodium_LIBRARY_RELEASE})
+  target_include_directories(cryptography_avx_lib PUBLIC ${sodium_INCLUDE_DIR})
+  target_link_libraries(${CRYPTOGRAPHY_LIB} PUBLIC ${sodium_LIBRARY_RELEASE})
 elseif(DOWNLOAD_SODIUM)
   message(STATUS "Sodium >= 1.0.17 not found, but DOWNLOAD_SODIUM specified, so downloading it")
   include(DownloadLibSodium)
-  target_link_libraries(${CRYPTOGRAPHY_LIB} sodium_vendor)
+  target_link_libraries(${CRYPTOGRAPHY_LIB} PUBLIC sodium_vendor)
 else()
   message(FATAL_ERROR "Could not find libsodium >= 1.0.17; either install it on your system or use -DDOWNLOAD_SODIUM=ON to download and build an internal copy")
 endif()
 
 target_include_directories(${CRYPTOGRAPHY_LIB} PUBLIC "${CMAKE_CURRENT_SOURCE_DIR}/libntrup/include")
+target_include_directories(cryptography_avx_lib PUBLIC "${CMAKE_CURRENT_SOURCE_DIR}/libntrup/include")