Warnings in GMock when compiling with -std=c++98

[vitaut]

https://travis-ci.org/cppformat/cppformat/jobs/114823247:

cppformat/test/gmock/gmock.h:1021:52: warning: invoking macro GMOCK_DEFINE_DEFAULT_ACTION_FOR_RETURN_TYPE_ argument 2: empty macro arguments are undefined in ISO C90 and ISO C++98 [enabled by default]
In file included from cppformat/test/format-impl-test.cc:37:0:
cppformat/test/gmock/gmock.h:4393:1: warning: invoking macro GTEST_CONCAT_TOKEN_IMPL_ argument 2: empty macro arguments are undefined in ISO C90 and ISO C++98 [enabled by default]
cppformat/test/gmock/gmock.h:4393:1: warning: invoking macro GTEST_CONCAT_TOKEN_IMPL_ argument 2: empty macro arguments are undefined in ISO C90 and ISO C++98 [enabled by default]
cppformat/test/gmock/gmock.h:4393:1: warning: invoking macro GTEST_CONCAT_TOKEN_IMPL_ argument 2: empty macro arguments are undefined in ISO C90 and ISO C++98 [enabled by default]
cppformat/test/gmock/gmock.h:4393:1: warning: invoking macro GTEST_CONCAT_TOKEN_IMPL_ argument 2: empty macro arguments are undefined in ISO C90 and ISO C++98 [enabled by default]
cppformat/test/gmock/gmock.h:4393:1: warning: invoking macro GTEST_CONCAT_TOKEN_IMPL_ argument 2: empty macro arguments are undefined in ISO C90 and ISO C++98 [enabled by default]
cppformat/test/gmock/gmock.h:4393:1: warning: invoking macro GTEST_CONCAT_TOKEN_IMPL_ argument 2: empty macro arguments are undefined in ISO C90 and ISO C++98 [enabled by default]
cppformat/test/gmock/gmock.h:4475:1: warning: invoking macro GTEST_CONCAT_TOKEN_IMPL_ argument 2: empty macro arguments are undefined in ISO C90 and ISO C++98 [enabled by default]
...

[vitaut]

Fixed in 0c901ef.