cmake: Disable SECP256K1_APPEND_*FLAGS options for VS generators

The "Visual Studio" generators create project files in a way that does
not use low-level rule variables. Consequently, setting
`SECP256K1_APPEND_*FLAGS` has no effect. Disabling these options ensures
that CMake warns the user if any of them are set.

CMakeLists.txt

@@ -280,19 +280,21 @@ if(SECP256K1_BUILD_CTIME_TESTS)
   unset(msan_enabled)
 endif()
 
-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)
-  # Appending to this low-level rule variable is the only way to
-  # guarantee that the flags appear at the end of the command line.
-  string(APPEND CMAKE_C_COMPILE_OBJECT " ${SECP256K1_APPEND_CFLAGS}")
-endif()
+if(NOT CMAKE_GENERATOR MATCHES "Visual Studio")
+  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)
+    # Appending to this low-level rule variable is the only way to
+    # guarantee that the flags appear at the end of the command line.
+    string(APPEND CMAKE_C_COMPILE_OBJECT " ${SECP256K1_APPEND_CFLAGS}")
+  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.")
-if(SECP256K1_APPEND_LDFLAGS)
-  # Appending to this low-level rule variable is the only way to
-  # guarantee that the flags appear at the end of the command line.
-  string(APPEND CMAKE_C_CREATE_SHARED_LIBRARY " ${SECP256K1_APPEND_LDFLAGS}")
-  string(APPEND CMAKE_C_LINK_EXECUTABLE " ${SECP256K1_APPEND_LDFLAGS}")
+  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.")
+  if(SECP256K1_APPEND_LDFLAGS)
+    # Appending to this low-level rule variable is the only way to
+    # guarantee that the flags appear at the end of the command line.
+    string(APPEND CMAKE_C_CREATE_SHARED_LIBRARY " ${SECP256K1_APPEND_LDFLAGS}")
+    string(APPEND CMAKE_C_LINK_EXECUTABLE " ${SECP256K1_APPEND_LDFLAGS}")
+  endif()
 endif()
 
 if(NOT CMAKE_RUNTIME_OUTPUT_DIRECTORY)
@@ -381,11 +383,13 @@ else()
   message(" - LDFLAGS for executables ............ ${CMAKE_EXE_LINKER_FLAGS_DEBUG}")
   message(" - LDFLAGS for shared libraries ....... ${CMAKE_SHARED_LINKER_FLAGS_DEBUG}")
 endif()
-if(SECP256K1_APPEND_CFLAGS)
-  message("SECP256K1_APPEND_CFLAGS ............... ${SECP256K1_APPEND_CFLAGS}")
-endif()
-if(SECP256K1_APPEND_LDFLAGS)
-  message("SECP256K1_APPEND_LDFLAGS .............. ${SECP256K1_APPEND_LDFLAGS}")
+if(NOT CMAKE_GENERATOR MATCHES "Visual Studio")
+  if(SECP256K1_APPEND_CFLAGS)
+    message("SECP256K1_APPEND_CFLAGS ............... ${SECP256K1_APPEND_CFLAGS}")
+  endif()
+  if(SECP256K1_APPEND_LDFLAGS)
+    message("SECP256K1_APPEND_LDFLAGS .............. ${SECP256K1_APPEND_LDFLAGS}")
+  endif()
 endif()
 message("")
 if(print_msan_notice)