Output error messages to the github summary file

This will show these messages in the Github Actions summary page to make
them more visible.

.github/workflows/make.yml

@@ -16,23 +16,32 @@ jobs:
       run: |
         clang-format -i */*.[ch]
         if git diff | grep ""; then
-          echo
-          echo "*** Formatting errors detected by clang-format."
-          echo "*** Please apply the above diff or run:"
-          echo "    clang-format -i */*.[ch]"
-          echo
+          (echo "Formatting errors detected by clang-format.";
+           echo "Please run `make format`.";
+           echo) >> $GITHUB_STEP_SUMMARY
           false
         fi
     - name: Check C89 compliance
       env:
         CFLAGS: "-O0 -Werror -std=c89 -pedantic -Isrc"
       run: |
-        gcc $CFLAGS -c */*.c
+        if ! gcc $CFLAGS -c */*.c; then
+          (echo "Code should be ANSI C. Make sure you are not using features";
+           echo "from later standards, including C++-style // comments.";
+           echo) >> $GITHUB_STEP_SUMMARY
+          false
+        fi
     - name: Check value types can be redefined
       env:
         CFLAGS: "-O0 -Werror -DTEST_ALTERNATE_VALUE_TYPES"
       run: |
-        gcc $CFLAGS -c src/*.c
+        if ! gcc $CFLAGS -c src/*.c
+          (echo "Key/value types cannot be redefined to something other than a"
+           echo "void pointer. Compile with -DTEST_ALTERNATE_VALUE_TYPES to see"
+           echo "the compile error."
+           echo) >> $GITHUB_STEP_SUMMARY
+          false
+        fi
 
   build_and_coverage:
     runs-on: ubuntu-latest