cmake: Fix passing SECP256K1_APPEND_CFLAGS to clang-cl

This change also makes the compile invocation string more natural by
ensuring flags do not follow source files.

CMakeLists.txt

@@ -287,7 +287,7 @@ if(CMAKE_GENERATOR MATCHES "Make|Ninja")
   # guarantee that the flags appear at the end of the command line.
   set(SECP256K1_APPEND_CFLAGS "" CACHE STRING "Compiler flags that are appended to the command line after all other flags added by the build system. This variable is intended for debugging and special builds.")
   if(SECP256K1_APPEND_CFLAGS)
-    string(APPEND CMAKE_C_COMPILE_OBJECT " ${SECP256K1_APPEND_CFLAGS}")
+    string(REPLACE "<FLAGS>" "<FLAGS> ${SECP256K1_APPEND_CFLAGS}" CMAKE_C_COMPILE_OBJECT "${CMAKE_C_COMPILE_OBJECT}")
   endif()
 
   set(SECP256K1_APPEND_LDFLAGS "" CACHE STRING "Linker flags that are appended to the command line after all other flags added by the build system. This variable is intended for debugging and special builds.")