AX_LIB_HDF5: remove extra space from include path flag

The code parsing the output of h5cc for the "Installation point:" results in an
extra leading space. The space creeps into HDF5_CPPFLAGS and breaks the include
path flag into two tokens, e.g., "-I" and "/usr/include". While this may be
legal and doesn't affect the compilation, it does break some library
tools (e.g., when the macro is used in building a dynamic library that uses
pkg-config).
NEWS: document recent changes
cfg.mk: copy 'README.md' to 'README' as part of the 'maintainer-all' target

Github wants the file to be called README.md, but the GNU coding
standards say that the file must be called README. Duh. So we compromise
by keeping the file README.md in the Git repository for Github, but
during the build process we make a suffix-less copy that is the one
we'll also distribute in the release archives.