build.sh: test that javac runs (macOS's /usr/bin/javac is only a placeholder)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

build.sh

@@ -17,7 +17,7 @@ if [ -z "${D8:-}" ] && [ -f "$CT/d8.jar" ]; then D8="java -cp $CT/d8.jar com.and
 D8=${D8:-d8}
 if [ -z "${APKTOOL:-}" ] && [ -f "$CT/apktool_2.9.3.jar" ]; then APKTOOL=$CT/apktool_2.9.3.jar; fi
 if [ -z "${APKSIGNER:-}" ] && [ -f "$CT/apksigner.jar" ]; then APKSIGNER=$CT/apksigner.jar; fi
-if ! command -v javac >/dev/null 2>&1; then
+if ! javac -version >/dev/null 2>&1; then   # macOS has a /usr/bin/javac that only says Java is missing
   for j in "${JAVA_HOME:-}" /opt/homebrew/opt/openjdk@17 /opt/homebrew/opt/openjdk; do
     if [ -n "$j" ] && [ -x "$j/bin/javac" ]; then JAVA_HOME=$j; PATH=$j/bin:$PATH; export JAVA_HOME PATH; break; fi
   done