Support per addon C++ flags

[fengli]

I'm trying to compile my module with C++ with additional flags, more
specific, -std=c++11.
I want to get my module use the standard nginx build scripts, like
auto/configure, etc.
However, looks like nginx only allows C++ options on global level. So, if I
specify the -std=c++11 in --with_cc_opt, other module may complain.
And it gets applied on other non-C++ files, like a .c files.

Unfortunately, nginx apply -Werror to stop on any warning, so, the build
process will stop at c files with error when -std=c++11 is given.

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,269147,269147#msg-269147