Fix macOS CI: use system cc instead of Homebrew gcc

Homebrew's gcc uses a different sysroot that lacks macOS-specific
symbols like reallocarray. Force CC=cc to use Apple's clang which
has the correct macOS SDK headers and library search paths.

Author: renaudallard

.github/workflows/build-and-fuzz.yml

@@ -114,6 +114,7 @@ jobs:
             BREW_PKG="${BREW_PKG:+${BREW_PKG}:}${prefix}/lib/pkgconfig"
           done
           ./configure --disable-dependency-tracking \
+            CC=cc \
             CPPFLAGS="${BREW_CFLAGS}" \
             LDFLAGS="${BREW_LDFLAGS}" \
             PKG_CONFIG_PATH="${BREW_PKG}"