Need to set options like --disable-lz4 at top level configure.ac as well as hdf5_plugins directory configure.ac

[edwardhartnett]

We are doing a bit of an odd thing because hdf5_plugins is a separate project, with its own configure.ac, and each of the filters is a separate project, with its own configure.ac.

We have some options like this in the hdf5_plugins level:

# Does the user want BZIP2?
AC_MSG_CHECKING([whether BZIP2 filter library should be built and installed])
AC_ARG_ENABLE([bzip2],
              [AS_HELP_STRING([--disable-bzip2],
                              [Disable the build and install of bzip2 filter library.])])
test "x$enable_bzip2" = xno || enable_bzip2=yes
AC_MSG_RESULT($enable_bzip2)
AM_CONDITIONAL(BUILD_BZIP2, [test "x$enable_bzip2" = xyes])

# Does the user want LZ4?
AC_MSG_CHECKING([whether LZ4 filter library should be built and installed])
AC_ARG_ENABLE([lz4],
              [AS_HELP_STRING([--disable-lz4],
                              [Disable the build and install of lz4 filter library.])])
test "x$enable_lz4" = xno || enable_lz4=yes
AC_MSG_RESULT($enable_lz4)
AM_CONDITIONAL(BUILD_LZ4, [test "x$enable_lz4" = xyes])

...

We need these options to be in the top-level configure.ac as well, so that we can use AM_CONDITIONALS like BUILD_LZ4 to decide whether or not to include the LZ4 library in the build and tests.