Update (and debundle) fmt dependancy

We now support the most recent released version of `fmt` available (at
the time
of committing).

As we no longer patch `fmt` to get it to work within `pyrogenesis`, this
commit
also mostly removes its source from our code-tree (some headers are
retained for
Windows builds).


If you are a user of...

Linux/BSD: You will now need to have `fmt` installed from your
distribution's
           package repository. The minimum supported version of `fmt` is
`4.0`.

      OSX: The source is acquired and compiled (in `build-osx-libs.sh`),
           then included and linked automatically.

  Windows: The relevant header files are retained and, along with a
pre-built
           library, are the only things still bundled.


Accepted by: wraitii
Tested by:
* Freagarach (Lubuntu 18.04, `fmt 4.0.0`)
* Krinkle (MacOS 10.14, `fmt 6.1.2`)
* nephele (Alpine Linux)
* wraitii (MacOS 10.14)
* Nescio (Fedora 33, `fmt 7.0.3`)

Windows library files built by: Stan
Fixes: #3190
Differential Revision: https://code.wildfiregames.com/D2689
This was SVN commit r24267.

libraries/LICENSE.txt

@@ -32,6 +32,9 @@ win32/ contains headers and precompiled static libs for Windows builds.
   win32/enet
     MIT
 
+  win32/fmt
+    MIT
+
   win32/gloox
     GPL v3
 

libraries/osx/build-osx-libs.sh

@@ -46,6 +46,7 @@ ICU_VERSION="icu4c-67_1"
 ENET_VERSION="enet-1.3.17"
 MINIUPNPC_VERSION="miniupnpc-2.1"
 SODIUM_VERSION="libsodium-1.0.18"
+FMT_VERSION="7.1.3"
 # --------------------------------------------------------------
 # Bundled with the game:
 # * SpiderMonkey
@@ -892,6 +893,49 @@ else
 fi
 popd > /dev/null
 
+# --------------------------------------------------------------
+echo -e "Building fmt..."
+
+LIB_DIRECTORY="fmt-$FMT_VERSION"
+LIB_ARCHIVE="$FMT_VERSION.tar.gz"
+LIB_URL="https://github.com/fmtlib/fmt/archive/"
+
+mkdir -p fmt
+pushd fmt > /dev/null
+
+if [[ "$force_rebuild" = "true" ]] || [[ ! -e .already-built ]] || [[ "$(<.already-built)" != "$FMT_VERSION" ]]
+then
+  rm -f .already-built
+
+  download_lib $LIB_URL $LIB_ARCHIVE
+
+  rm -rf $LIB_DIRECTORY include lib
+  tar -xf $LIB_ARCHIVE
+  pushd $LIB_DIRECTORY
+
+  # It appears that older versions of Clang require constexpr statements to have a user-set constructor.
+  patch -Np1 -i ../../patches/fmt_constexpr.diff
+
+  mkdir -p build
+  pushd build
+
+  (cmake .. \
+      -DFMT_TEST=False \
+      -DFMT_DOC=False \
+    && make fmt ${JOBS}) || die "fmt build failed"
+  popd
+
+  mkdir -p ../lib
+  cp build/libfmt.a ../lib/
+  cp -r include ../include
+
+  popd
+  echo "$FMT_VERSION" > .already-built
+else
+  already_built
+fi
+popd > /dev/null
+
 # --------------------------------------------------------------------
 # The following libraries are shared on different OSes and may
 # be customized, so we build and install them from bundled sources

libraries/osx/patches/fmt_constexpr.diff

@@ -0,0 +1,13 @@
+diff --git a/include/fmt/os.h b/include/fmt/os.h
+index 88151006..11c3b2cc 100644
+--- a/include/fmt/os.h
++++ b/include/fmt/os.h
+@@ -375,7 +375,7 @@ struct ostream_params {
+ };
+ }  // namespace detail
+ 
+-static constexpr detail::buffer_size buffer_size;
++static constexpr detail::buffer_size buffer_size{};
+ 
+ // A fast output stream which is not thread-safe.
+ class ostream final : private detail::buffer<char> {