cmake: Enable APPEND_*FLAGS only for command-line generators

Command-line build tool generators are the only ones that support
low-level rule variables. IDE build tool generators, such as "Visual
Studio" and "Xcode", do not utilize these variables when creating
project files.

This change ensures that CMake warns users if any of the
`SECP256K1_APPEND_*FLAGS` options are set while using an unsupported
generator.

CMakeLists.txt

@@ -280,19 +280,20 @@ 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
+# Only some generators use low-level rule variables.
+if(CMAKE_GENERATOR MATCHES "Make|Ninja")
+  # Appending to the low-level rule variables 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_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}")
+  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)
+    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 +382,13 @@ 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(CMAKE_GENERATOR MATCHES "Make|Ninja")
+  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)