cmake: Add APPEND_{CPP,C,CXX,LD}FLAGS cache variables

The content of those variables is appended to the each target flags
added by the build system.

CMakeLists.txt

@@ -140,6 +140,13 @@ cmake_dependent_option(FUZZ "Build for fuzzing. Enabling this will disable all o
 
 option(INSTALL_MAN "Install man pages." ON)
 
+set(APPEND_CPPFLAGS "" CACHE STRING "Preprocessor flags that are appended to the flags added by the build system.")
+set(APPEND_CFLAGS "" CACHE STRING "C compiler flags that are appended to the flags added by the build system.")
+set(APPEND_CXXFLAGS "" CACHE STRING "(Objective) C++ compiler flags that are appended to the flags added by the build system.")
+set(APPEND_LDFLAGS "" CACHE STRING "Linker flags that are appended to the flags added by the build system.")
+string(APPEND CMAKE_CXX_COMPILE_OBJECT " ${APPEND_CPPFLAGS} ${APPEND_CXXFLAGS}")
+string(APPEND CMAKE_CXX_LINK_EXECUTABLE " ${APPEND_LDFLAGS}")
+
 set(configure_warnings)
 
 include(CheckPIESupported)

cmake/optional_qt.cmake

@@ -40,6 +40,7 @@ if(WITH_GUI)
         set(CMAKE_OBJCXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE}")
         set(CMAKE_OBJCXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG}")
         set(CMAKE_OBJCXX_FLAGS_MINSIZEREL "${CMAKE_CXX_FLAGS_MINSIZEREL}")
+        string(APPEND CMAKE_OBJCXX_COMPILE_OBJECT " ${APPEND_CPPFLAGS} ${APPEND_CXXFLAGS}")
       endif()
     else()
       message(WARNING "Qt not found, disabling.\n"

cmake/secp256k1.cmake

@@ -9,6 +9,7 @@
 enable_language(C)
 set(CMAKE_C_STANDARD 90)
 set(CMAKE_C_EXTENSIONS OFF)
+string(APPEND CMAKE_C_COMPILE_OBJECT " ${APPEND_CPPFLAGS} ${APPEND_CFLAGS}")
 
 include(CheckCSourceCompiles)
 check_c_source_compiles("