Merge #282: cmake: Allow overriding flags in compiler/linker flag checks

ed471c5 fixup! cmake: Add `APPEND_{CPP,C,CXX,LD}FLAGS` cache variables (Hennadii Stepanov)

Pull request description:

  The `APPEND_{CPP,C,CXX,LD}FLAGS` flags are now considered when checking compiler/linker flags.

  Fixes #279.

Top commit has no ACKs.

Tree-SHA512: 217ab660508d19bcf8196833c05cc75c1439d1685e8f2718ddaa9248c4964a8d0e9d1e37bf2b08ce1e20aa2fc5d2a0faf532a8e20d71d7ac3d5f025f07c2b6d3

CMakeLists.txt

@@ -26,6 +26,22 @@ set(CLIENT_VERSION_RC 0)
 set(CLIENT_VERSION_IS_RELEASE "false")
 set(COPYRIGHT_YEAR "2024")
 
+# During the enabling of the CXX and CXXOBJ languages, we modify
+# CMake's compiler/linker invocation strings by appending the content
+# of the user-defined `APPEND_*` variables, which allows overriding
+# any flag. We also ensure that the APPEND_* flags are considered
+# during CMake's tests, which use the `try_compile()` command.
+#
+# CMake's docs state that the `CMAKE_TRY_COMPILE_PLATFORM_VARIABLES`
+# variable "is meant to be set by CMake's platform information modules
+# for the current toolchain, or by a toolchain file." We do our best
+# to set it before the `project()` command.
+set(CMAKE_TRY_COMPILE_PLATFORM_VARIABLES
+  CMAKE_CXX_COMPILE_OBJECT
+  CMAKE_OBJCXX_COMPILE_OBJECT
+  CMAKE_CXX_LINK_EXECUTABLE
+)
+
 project(BitcoinCore
   VERSION ${CLIENT_VERSION_MAJOR}.${CLIENT_VERSION_MINOR}.${CLIENT_VERSION_BUILD}
   DESCRIPTION "Bitcoin client software"