C11emu is an emulation layer/library for the C11 (C 2011) Concurrency Support Library which has been slow to be implemented on many systems.
It provides functions like the tss_create() family of thread-local storage routines.
The code is based on @yohhoy's pure headerfile implementation as it was included in the Mesa project: see https://gist.github.com/yohhoy/2223710
These headers are installed under $prefix/include/c11emu/header_only. Projects that include, for instance, thread.h and use the functions declared therein in a single or small number of source files can use CFLAGS += -I/path/to/include/c11emu/header_only if the system doesn't provide the thread.h headerfile itself.
Recent versions of Mesa have moved from a pure headerfile implementation to one using a library. This approach is more suitable for projects where the provided functions are called from multiple source files. It will be added in an upcoming version.
The original implementation by @yohhoy was covered by the Boost (Permissive) license; the library re-implementation by Mesa's usual MIT license.