Fix JAVA detection for MSVC and support white space in Java path. * configure.ac (JAVA_HOME, JAVA_PATH, java_version): Quote java-related paths and commands and consider backslash in path manipulations. Also detect the case of MSYS shell (mingw canonical host) with MSVC. In MSYS shell, convert JAVA_HOME into MSYS path (no backslash or colon). * scripts/java/modules.mk: Double-quote usage of $(JAVAC) and $(JAR).