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.

source/maths/Fixed.h

@@ -146,7 +146,7 @@ public:
 
 	static CFixed FromFloat(float n)
 	{
-		if (!isfinite(n))
+		if (!std::isfinite(n))
 			return CFixed(0);
 		float scaled = n * fract_pow2;
 		return CFixed(round_away_from_zero<T>(scaled));
@@ -154,7 +154,7 @@ public:
 
 	static CFixed FromDouble(double n)
 	{
-		if (!isfinite(n))
+		if (!std::isfinite(n))
 			return CFixed(0);
 		double scaled = n * fract_pow2;
 		return CFixed(round_away_from_zero<T>(scaled));