Update cflags include to use pkg-config if include directory is different then default /usr/include

[rkalkani]

We are using think package in an embedded rootfs and we are not able to build this package within Yocto recipe since the includes are from different.

We can update the build to also respect the pkg-config value on Linux.

[rkalkani]

@sebastianarena Hi, We love your package and are thankful for your work. Will you please have a look at PR. I did not find the contribution guide so not sure If I opened PR correctly or not. Let me know your feedback on PR, and what we need to update to have these changes on upstream instead of managing our own fork.

We are using your library it in an embedded system. The current hardcoded include path does not work when doing the cross-platform build. We could either add extra include flag to get right include path from pkg-config OR replace fix include path with pkg-config. I think pkg-config is nowadays available on most of the distros.

[santigimeno]

@rkalkani sorry this PR fell through the cracks. Let me take a look at it and if it's fine I'll merge it.

[bkchr]

@santigimeno any update on this? :D