…ports for Darwin targets

de4238f build: consolidate reduced export checks (fanquake)
012bdec build: add building libconsensus to end-of-configure output (fanquake)
8f360e3 build: remove ax_gcc_func_attribute macro (fanquake)
f054a08 build: remove AX_GCC_FUNC_ATTRIBUTE test for dllimport (fanquake)
7cd0a69 build: test for __declspec(dllexport) in configure (fanquake)
1624e17 build: remove duplicate visibility attribute detection (fanquake)

Pull request description:

  Darwin targets do not have a `protected` visibility function attribute, see [LLVM explanation](https://github.com/llvm/llvm-project/blob/8e9a505139fbef7d2e6e9d0adfe1efc87326f9ef/clang/lib/Basic/Targets/OSTargets.h#L131). This means that the `AX_GCC_FUNC_ATTRIBUTE` check for `visibility` fails:
  ```bash
  configure:24513: checking for __attribute__((visibility))
  configure:24537: g++ -std=c++11 -o conftest -g -O2  -DHAVE_BUILD_INFO -D__STDC_FORMAT_MACROS -DMAC_OSX -DOBJC_OLD_DISPATCH_PROTOTYPES=0  -Wl,-headerpad_max_install_names conftest.cpp  >&5
  conftest.cpp:35:56: warning: target does not support 'protected' visibility; using 'default' [-Wunsupported-visibility]
                      int foo_pro( void ) __attribute__((visibility("protected")));
                                                         ^
  1 warning generated.
  configure:24537: $? = 0
  configure:24550: result: no
  ```

  This leads to `EXPORT_SYMBOL` being [defined to nothing](https://github.com/bitcoin/bitcoin/blob/f4de89edfa8be4501534fec0c662c650a4ce7ef2/src/script/bitcoinconsensus.h#L29), as `HAVE_FUNC_ATTRIBUTE_VISIBILITY` is not defined, and when building with reduced exports, you end up with a libbitcoinconsensus.dylib that doesn't export any  `_bitcoinconsensus_*` symbols.
  ```bash
  ➜  git:(master) nm -C src/.libs/libbitcoinconsensus.dylib | rg _bitcoinconsensus_
  ➜  git:(master)
  ```

  We do have a [second check](https://github.com/bitcoin/bitcoin/blob/f4de89edfa8be4501534fec0c662c650a4ce7ef2/configure.ac#L882) for the `visibility` attribute, which works for Darwin as it's only testing for default visibility, however the result of this check isn't used at all. It was added in bitcoin#4725, along with the `--enable-reduce-exports` option, however when libbitcoinconsensus was added in bitcoin#5235, it used the results of the added `AX_GCC_FUNC_ATTRIBUTE` calls.

  This PR removes our usage of the AX_GCC_FUNC_ATTRIBUTE macro entirely, in favour of our own checks in configure. This meant adding a check for `dllexport`, which I've tested as working with both [GCC](https://gcc.gnu.org/onlinedocs/gcc/Microsoft-Windows-Function-Attributes.html) and [Clang](https://releases.llvm.org/10.0.0/tools/clang/docs/AttributeReference.html#dllexport) when building for Windows. I haven't added an equivalent check for `dllimport`, as we weren't actually using the result of that check, we're just testing that `MSC_VER` was defined before using.

  With these changes building a libbitcoinconsensus with reduced exports, when targeting Darwin, works as expected:
  ```bash
  ./autogen.sh
  ./configure --disable-tests --disable-bench --with-utils=no --with-daemon=no --with-gui=no --disable-wallet --with-libs=yes --enable-reduce-exports
  make -j8
  ...
  nm -C src/.libs/libbitcoinconsensus.dylib | rg _bitcoinconsensus_
  000000000000a340 T _bitcoinconsensus_verify_script
  00000000000097e0 T _bitcoinconsensus_verify_script_with_amount
  000000000000a3c0 T _bitcoinconsensus_version
  ```

  ```python
  >>> import ctypes
  >>> consensus = ctypes.CDLL("src/.libs/libbitcoinconsensus.dylib")
  >>> print(consensus.bitcoinconsensus_version())
  1
  >>> exit()
  ```

  TODO: Modify a CI job to compile with --enable-reduce-exports and check for symbols in shared lib?

ACKs for top commit:
  laanwj:
    Code review ACK de4238f

Tree-SHA512: d148f3c55d14dac6e9e5b718cc65bb557bcf6f663218d24bc9044b86281bd5dd3d931ebea79c336a58e8ed50d683218c0a9e75494f2267b91097665043e252ae