GLSLANG_EXPORT for C APIs.

[ezdiy]

Fixes FFI breakage introduced in #2283

[CLAassistant]

All committers have signed the CLA.

[johnkslang]

We normally have 11 or twelve checks, but this only saw 2. This PR though is a good candidate to see running on more bots, somehow, before merging.

@ben-clayton is taking a break right now, but would also be nice to have his input on the change itself.

[ben-clayton]

@johnkslang - you need to apply the "kokoro: run" label for the other bots to run.
The change LGTM.

[ezdiy]

One issue currently not taken into account is https://github.com/KhronosGroup/glslang/blob/master/StandAlone/resource_limits_c.h

This compiles into libglslang-default-resource-limits.so currently not under -fvisibility=hidden regime in the makefiles, hence exports still working (at least on linux, not so sure about win32). Making it export properly would ideally involve collapsing all that into glslang_c_interface.c / glslang_c_interface.h, as the separation to additional library looks to me like some temporary workaround to deal with circular dependency yet to be refactored. Alternatively turn whole C api into separate library that has its own linking mode - ie even if c++ is static, c-export may be dynamic, and perhaps make that a cmake default.

The reasoning here is that FFI from non-C languages (think LWJGL...) demand shared c-export library, yet don't care about nature of t he c++ side whatsoever.

[johnkslang]

Can you say more about the "kokoro: run" label?

(This project doesn't have such a label, and normally all tests run automatically.)