diff --git a/.github/workflows/unix.yml b/.github/workflows/unix.yml
index 926d10b..250e3a0 100644
--- a/.github/workflows/unix.yml
+++ b/.github/workflows/unix.yml
@@ -5,20 +5,16 @@ env:
   PKG_CONFIG_PATH: /usr/local/share/pkgconfig
 
 jobs:
-  c_macos:
-    name: macos-12 / C
-    runs-on: macos-12
-    steps:
-      - uses: actions/checkout@v4
-      - run: ./run.sh
-      - run: cd tests && CC=gcc-13 make && ./tests
-  c_linux:
-    name: ubuntu-22.04 / C
-    runs-on: ubuntu-22.04
+  c:
+    name: ${{ matrix.os }} / C
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [macos-12, ubuntu-22.04]
     steps:
       - uses: actions/checkout@v4
       - run: ./run.sh
-      - run: cd tests && make && ./tests
+      - run: cd tests && CC=gcc-11 make && ./tests
   python:
     name: ${{ matrix.os }} / Python
     runs-on: ${{ matrix.os }}
diff --git a/tests/tests.c b/tests/tests.c
index e1e1cce..dc83095 100644
--- a/tests/tests.c
+++ b/tests/tests.c
@@ -5,7 +5,12 @@
 #include <stddef.h>
 #include <stdio.h>
 
-#ifndef __STDC_NO_THREADS__
+// The C compilers available on the macOS runners on GitHub Actions do not
+// indicate their lack of support for standard threads with the expected
+// preprocessor macro, so disable multithreading on macOS.
+#if defined __APPLE__ || defined __STDC_NO_THREADS__
+#define STDC_NO_THREADS
+#else
 #include <threads.h>
 #endif
 
@@ -74,12 +79,12 @@ main(void)
         hdrbg_tests(hds[i], tv);
         rewind(tv);
     }
-#ifndef __STDC_NO_THREADS__
+#ifndef STDC_NO_THREADS
     thrd_t workers[WORKERS_SIZE];
 #endif
     for (int i = 0; i < WORKERS_SIZE; ++i)
     {
-#ifndef __STDC_NO_THREADS__
+#ifndef STDC_NO_THREADS
         thrd_create(workers + i, hdrbg_tests_custom, hds[i]);
 #else
         hdrbg_tests_custom(hds[i]);
@@ -87,7 +92,7 @@ main(void)
     }
     for (int i = 0; i < WORKERS_SIZE; ++i)
     {
-#ifndef __STDC_NO_THREADS__
+#ifndef STDC_NO_THREADS
         thrd_join(workers[i], NULL);
 #endif
         hdrbg_zero(hds[i]);