config/opal_setup_java.m4: Fix #5015.

That PR accidentally changed Open MPI's build configuration infrastruc-
ture's Java toolchain detection logic so that it would, as reported by @bosilca
in https://github.com/open-mpi/ompi/pull/5001#issuecomment-387803012 and tracked down by me in https://github.com/open-mpi/ompi/pull/5001#issuecomment-387851005, abort your entire
in-progress Open MPI build when it failed to find an OS X/macOS JDK instead of
simply falling back to checking for a JDK in locations where it would be found
on other platforms.  _Oops…!_

Signed-off-by: Bryce Glover <RandomDSdevel@gmail.com>

config/opal_setup_java.m4

@@ -113,8 +113,8 @@ AC_DEFUN([OPAL_SETUP_JAVA],[
                           with_jdk_headers=$opal_java_dir/include
                           with_jdk_bindir=$opal_java_dir/bin
                       else
-                          AC_MSG_WARN([No recognized directory structure found under $opal_java_dir])
-                          AC_MSG_ERROR([Cannot continue])
+                          AC_MSG_WARN([No recognized OS X/macOS JDK directory structure found under $opal_java_dir])
+                          opal_java_found=0
                       fi],
                      [AC_MSG_RESULT([not found])])