cmake: Rename SECP256K1_LATE_CFLAGS to SECP256K1_APPEND_CFLAGS

This change follows the naming convention currently used in the Bitcoin
Core's CMake staging branch.

CMakeLists.txt

@@ -284,14 +284,14 @@ if(SECP256K1_BUILD_BENCHMARK OR SECP256K1_BUILD_TESTS OR SECP256K1_BUILD_EXHAUST
   enable_testing()
 endif()
 
-set(SECP256K1_LATE_CFLAGS "" CACHE STRING "Compiler flags that are added to the command line after all other flags added by the build system.")
+set(SECP256K1_APPEND_CFLAGS "" CACHE STRING "Compiler flags that are added to the command line after all other flags added by the build system.")
 include(AllTargetsCompileOptions)
 
 add_subdirectory(src)
-all_targets_compile_options(src "${SECP256K1_LATE_CFLAGS}")
+all_targets_compile_options(src "${SECP256K1_APPEND_CFLAGS}")
 if(SECP256K1_BUILD_EXAMPLES)
   add_subdirectory(examples)
-  all_targets_compile_options(examples "${SECP256K1_LATE_CFLAGS}")
+  all_targets_compile_options(examples "${SECP256K1_APPEND_CFLAGS}")
 endif()
 
 message("\n")
@@ -365,8 +365,8 @@ else()
   message(" - LDFLAGS for executables ............ ${CMAKE_EXE_LINKER_FLAGS_DEBUG}")
   message(" - LDFLAGS for shared libraries ....... ${CMAKE_SHARED_LINKER_FLAGS_DEBUG}")
 endif()
-if(SECP256K1_LATE_CFLAGS)
-  message("SECP256K1_LATE_CFLAGS ................. ${SECP256K1_LATE_CFLAGS}")
+if(SECP256K1_APPEND_CFLAGS)
+  message("SECP256K1_APPEND_CFLAGS ............... ${SECP256K1_APPEND_CFLAGS}")
 endif()
 message("")
 if(print_msan_notice)