cmake: only use -fno-strict-aliasing on files requiring it

[illwieckz]

This flag is there since first commit and it is likely no one remembers why it has been set to begin with.

I experience no difference when removing this.

This was only used on engine build and dll/exe game. This was never used on nexe game.

[illwieckz]

See #1158:

• Why do we build engine and non-nexe game with -fno-strict-aliasing? #1158

[illwieckz]

So now we know:

engine/qcommon/msg.cpp: In function ‘void MSG_WriteDeltaFloat(msg_t*, float, float)’:
engine/qcommon/msg.cpp:626:24: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
  626 |  MSG_WriteBits( msg, * ( int * ) &newV, 32 );
      |                        ^~~~~~~~~~~~~~~
engine/qcommon/msg.cpp: In function ‘float MSG_ReadDeltaFloat(msg_t*, float)’:
engine/qcommon/msg.cpp:635:5: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
  635 |   * ( int * ) &newV = MSG_ReadBits( msg, 32 );
      |     ^~~~~~~~~~~~~~~

[illwieckz]

Now we only use the disablement flag to build the only three functions requiring it. All other parts of code are now built with the optimization.

This seems to be ready.

[illwieckz]

Closed in favor of doing the opposite (always using that flag):

• cmake: never use strict aliasing #1160