[BUILD] Added macros resolved in srt.h to pkg-config file

[ethouris]

Fixes #2610

Replaces #2696

There are several preprocessor macros that are used as conditionals in srt.h; they should be always resolved before including this file.

In order to achieve this, you should add the result of pkg-config --cflags srt to your applications' compile command line options, just as well the result of pkg-config --libs srt to your applications' linker options. This will provide all required options your application should be compiled with, including macrodefinitions with which the library was originally compiled.

[jeandube]

pkg-config --cflags srt is not terrible for cross compiling. First it adds -I /usr/local/include and second, the install dir of a cross-compiled SRT is rarely the build system's PKG_CONFIG_PATH to which most developers don't have access anyway.

[ethouris]

How do you get access to libssl then? This is also using pkg-config in the SRT build. This will likely have to use the right version of pkg-config from the cross-compile toolchain. I found something about it here

[jeandube]

openssl is built and installed by the maintainer of the build computer. SRT is kind of part of the firmware build

[ethouris]

Ok, so there's an alternative, enabled with -DENABLE_CONFIG_HEADER=ON.

If enabled, these flags will be placed in the generated version.h file. This file is automatically included by srt.h, so if configured this way, you don't have to do anything.

The controversy is that every user that is including the SRT header will have to use pkg-config by default, or request this flag otherwise.