gRPC C++ will drop gcc 4.9 support from 1.45

[veb...@google.com]

gRPC C++ will change the minimum supported version of gcc from 1.45. Currently it is 4.9+ but it will change to 5.1+. This is decided to be in line with the dependencies of gRPC such as Abseil and GoogleTest which require gcc 5+. From 1.45, the supported compilers of gRPC C++ will be

• GCC 5.1+
• Clang 4+
• Visual Studio 2015+

Thanks!