test: C compilation tests for generated template code

- Add gcc and mesa to shell.nix for C compilation
- Fix warning icon overlap with ::after message by adding padding-right
  to proccall-header and end-header when warnings are active
- Add tests/compile/ with Makefile (pkg-config based) and run.js
  which generates C source from block-like data structures, then
  compiles with gcc -Wall -Wextra -fsyntax-only
- Tests cover: console template, simple error, error merge,
  sequential errors, value-return error style

style.css

@@ -635,6 +635,9 @@ wc-proccall.warn-error-unlinked {
   box-shadow: 0 0 0 2px #ef5350, 0 1px 3px rgba(0,0,0,0.08);
 }
 wc-proccall.warn-error-unlinked > .block-warn { display: inline; color: #ef5350; }
+wc-proccall.warn-error-unlinked .proccall-header {
+  padding-right: 1.2rem;
+}
 wc-proccall.warn-error-unlinked .proccall-header::after {
   content: " (unused)"; font-size: 0.65rem; color: #ef5350; font-weight: normal;
 }
@@ -646,6 +649,9 @@ wc-end.warn-zero-in-error {
   background: #fff8e1;
 }
 wc-end.warn-zero-in-error > .block-warn { display: inline; color: #ffa000; }
+wc-end.warn-zero-in-error .end-header {
+  padding-right: 1.8rem;
+}
 wc-end.warn-zero-in-error .end-header::after {
   content: " (success in error path)";
   font-size: 0.6rem; color: #ffa000; font-weight: normal;