cmake: Set macOS target for blst only if understood (#1028)

In some configurations (e.g. Silkworm / conan) the
`CMAKE_OSX_DEPLOYMENT_TARGET` may be defined even if we don't target
macOS. Pass this value to the C compiler only if it supports the
deployment target flag (`-mmacos-version-min=`).

Author: chfast

cmake/blst.cmake

@@ -9,7 +9,7 @@ else()
     if(CMAKE_OSX_SYSROOT)
         set(BLST_CC "${BLST_CC} ${CMAKE_C_SYSROOT_FLAG} ${CMAKE_OSX_SYSROOT}")
     endif()
-    if(CMAKE_OSX_DEPLOYMENT_TARGET)
+    if(CMAKE_C_OSX_DEPLOYMENT_TARGET_FLAG AND CMAKE_OSX_DEPLOYMENT_TARGET)
         set(BLST_CC "${BLST_CC} ${CMAKE_C_OSX_DEPLOYMENT_TARGET_FLAG}${CMAKE_OSX_DEPLOYMENT_TARGET}")
     endif()
     if(CMAKE_C_FLAGS)