Bug: Compilation fails with gcc 15 without threading enabled

[jcarrete5]

Problem/Opportunity

Building tasecureapi with a compiler that doesn't support threads.h fails, despite the existence of a header under reference/include/ to provide this file if it's missing.

$ cmake --build build-gcc/
[112/128] Building C object src/client/CMakeFiles/saclient.dir/src/sa_provider.c.o
FAILED: [code=1] src/client/CMakeFiles/saclient.dir/src/sa_provider.c.o
/opt/homebrew/bin/gcc-15 -Dsaclient_EXPORTS -I/Users/jcarre110/Git/tasecureapi/reference/src/client/include -I/Users/jcarre110/Git/tasecureapi/reference/src/client/../util/include -I/Users/jcarre110/Git/tasecureapi/reference/src/client/src -I/opt/homebrew/Cellar/openssl@3/3.6.1/include -I/Users/jcarre110/Git/tasecureapi/reference/src/util/include -I/Users/jcarre110/Git/tasecureapi/reference/src/util/../client/include -D_GNU_SOURCE -fPIC  -g -std=gnu11 -arch arm64 -fPIC -Werror -Wall -Wextra -Wno-type-limits -Wno-unused-parameter -Wno-deprecated-declarations -MD -MT src/client/CMakeFiles/saclient.dir/src/sa_provider.c.o -MF src/client/CMakeFiles/saclient.dir/src/sa_provider.c.o.d -o src/client/CMakeFiles/saclient.dir/src/sa_provider.c.o -c /Users/jcarre110/Git/tasecureapi/reference/src/client/src/sa_provider.c
/Users/jcarre110/Git/tasecureapi/reference/src/client/src/sa_provider.c:33:10: fatal error: threads.h: No such file or directory
   33 | #include <threads.h>
      |          ^~~~~~~~~~~
compilation terminated.
[123/128] Building C object src/client/CMakeFiles/saclient.dir/src/sa_provider_keymgt.c.o
ninja: build stopped: subcommand failed.

Steps to reproduce

1. Install gcc from homebrew: brew install gcc
2. CC=gcc-15 CXX=g++-15 cmake -B build-gcc -S reference
3. cmake --build build-gcc

Expected Behavior

Program should compile.

Actual Behavior

Program doesn't compile.

Notes (Optional)

Should be able to resolve by add reference/include to the include directories when compiling sources under reference/src/client.