C++ warning when testing with testthat

[DavisVaughan]

On Windows with R 4.2.1, testing cpp11 throws these warnings that come from testthat:

D:/a/_temp/Library/testthat/include/testthat/testthat.h: In function 'std::ostream& Catch::cout()':
D:/a/_temp/Library/testthat/include/testthat/testthat.h:145:1: warning: visibility attribute not supported in this configuration; ignored [-Wattributes]
  145 | }
      | ^

D:/a/_temp/Library/testthat/include/testthat/testthat.h: In function 'std::ostream& Catch::cerr()':
D:/a/_temp/Library/testthat/include/testthat/testthat.h:152:1: warning: visibility attribute not supported in this configuration; ignored [-Wattributes]
  152 | }
      | ^

It is pointing to the usage of TESTTHAT_ATTRIBUTE_HIDDEN in these two functions

testthat/inst/include/testthat/testthat.h

Lines 140 to 152 in 426eb52

	TESTTHAT_ATTRIBUTE_HIDDEN
	inline std::ostream& cout()
	{
	static testthat::r_ostream instance;
	return instance;
	}
	TESTTHAT_ATTRIBUTE_HIDDEN
	inline std::ostream& cerr()
	{
	static testthat::r_ostream instance;
	return instance;
	}

Here is the definition of that macro:

testthat/inst/include/testthat/testthat.h

Lines 20 to 25 in 426eb52

	#if defined(__GNUC__) || defined(__clang__)
	# define TESTTHAT_ENABLED
	# define TESTTHAT_ATTRIBUTE_HIDDEN __attribute__ ((visibility("hidden")))
	#else
	# define TESTTHAT_ATTRIBUTE_HIDDEN
	#endif

And here is the original PR where it was added 5d052fc. The original problem that prompted the addition of this macro was for Linux, and wasn't a problem on Windows, so I feel like tweaking this for Windows is safe.

Apparently __attribute__ ((visibility("hidden"))) isn't supported on Windows with the version of gcc that comes with Rtools + R 4.2.1?

This doesn't happen on R 3.6, so it probably has to do with the updated Rtools and version of gcc.

It seems to be a general issue with the MinGW version of gcc, other people worked around it by just not using the attribute, so I feel like we should do that too fmtlib/fmt#1743

[DavisVaughan]

 #if (defined(__GNUC__) || defined(__clang__)) && !defined(__MINGW32__)
 # define TESTTHAT_ATTRIBUTE_HIDDEN __attribute__ ((visibility("hidden"))) 
 #else 
 # define TESTTHAT_ATTRIBUTE_HIDDEN 
 #endif 

Maybe pull the definition of that macro out into something like this