Add -Wmissing-declarations

[jbytheway]

Summary

SUMMARY: Infrastructure "Compile with -Wmissing-declarations by default"

Purpose of change

This warning helps keep code tidy and comprehensible by ensuring functions local to a single file are marked static. This offers:

• Clarity for devs.
• Allows you to know when they are unused and can be deleted.
• Helps the optimizer make better choices.
• Smaller export tables in libraries.

Describe the solution

Enable -Wmissing-declarations and tidy up remaining failures related to that warning.

Add to Makefile and CMakeLists.txt

[jbytheway]

Looks like I forgot to check the json formatter code...

[kevingranade]

Well there's a point against moving the win curses build out of the PR build list :/

src/wincurse.cpp:65:14: error: no previous declaration for 'std::__cxx11::wstring widen(const string&)' [-Werror=missing-declarations]
 std::wstring widen( const std::string &s )
              ^
src/wincurse.cpp: In function 'bool WinCreate()':
src/wincurse.cpp:79:6: error: no previous declaration for 'bool WinCreate()' [-Werror=missing-declarations]
 bool WinCreate()
      ^
src/wincurse.cpp: In function 'void WinDestroy()':
src/wincurse.cpp:131:6: error: no previous declaration for 'void WinDestroy()' [-Werror=missing-declarations]
 void WinDestroy()
      ^
src/wincurse.cpp: In function 'void create_backbuffer()':
src/wincurse.cpp:145:6: error: no previous declaration for 'void create_backbuffer()' [-Werror=missing-declarations]
 void create_backbuffer()
      ^
src/wincurse.cpp: In function 'void add_alt_code(char)':
src/wincurse.cpp:210:6: error: no previous declaration for 'void add_alt_code(char)' [-Werror=missing-declarations]
 void add_alt_code( char c )
      ^
src/wincurse.cpp: In function 'void CheckMessages()':
src/wincurse.cpp:537:6: error: no previous declaration for 'void CheckMessages()' [-Werror=missing-declarations]
 void CheckMessages()
      ^
src/wincurse.cpp: In function 'uint64_t GetPerfCount()':
src/wincurse.cpp:639:10: error: no previous declaration for 'uint64_t GetPerfCount()' [-Werror=missing-declarations]
 uint64_t GetPerfCount()
          ^
cc1plus: all warnings being treated as errors
Makefile:768: recipe for target 'objwin/wincurse.o' failed
make: *** [objwin/wincurse.o] Error 1
make: *** Waiting for unfinished jobs....

[jbytheway]

Looks promising. Gorgon complaint was just due to the Makefile change.