hdf5

[svn-r26262] Restores maintainer mode to the configure.ac file but leaves it on
by default (unlike the trunk and 1.8 branches, where it's disabled
by default).

Users can build with --disable-maintainer-mode to remove autotools
build dependencies.

Part of: HDFFV-9122

Tested on: 64-bit linux VM (configure and autogen only)