cmake: Move CMAKE_C_FLAGS deduplication after enabling C language

CMakeLists.txt

@@ -587,11 +587,8 @@ target_compile_definitions(core_interface INTERFACE ${DEPENDS_COMPILE_DEFINITION
 target_compile_definitions(core_interface_relwithdebinfo INTERFACE ${DEPENDS_COMPILE_DEFINITIONS_RELWITHDEBINFO})
 target_compile_definitions(core_interface_debug INTERFACE ${DEPENDS_COMPILE_DEFINITIONS_DEBUG})
 
-# If {C,CXX,LD}FLAGS variables are defined during building depends and
-# configuring this build system, their content might be duplicated.
-if(DEFINED ENV{CFLAGS})
-  deduplicate_flags(CMAKE_C_FLAGS)
-endif()
+# If the {CXX,LD}FLAGS environment variables are defined during building depends
+# and configuring this build system, their content might be duplicated.
 if(DEFINED ENV{CXXFLAGS})
   deduplicate_flags(CMAKE_CXX_FLAGS)
 endif()

src/CMakeLists.txt

@@ -66,6 +66,11 @@ foreach(config IN LISTS CMAKE_BUILD_TYPE CMAKE_CONFIGURATION_TYPES)
   string(TOUPPER "${config}" config)
   set(CMAKE_C_FLAGS_${config} "${CMAKE_C_FLAGS_RELWITHDEBINFO}")
 endforeach()
+# If the CFLAGS environment variable is defined during building depends
+# and configuring this build system, its content might be duplicated.
+if(DEFINED ENV{CFLAGS})
+  deduplicate_flags(CMAKE_C_FLAGS)
+endif()
 set(CMAKE_EXPORT_COMPILE_COMMANDS OFF)
 add_subdirectory(secp256k1)
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)