build system: Introduce INSTALL_SHLIB and SYMLINK_SHLIB

build system: Introduce INSTALL_SHLIB and SYMLINK_SHLIB

INSTALL_SHLIB is like INSTALL_PROG but used only for shared libraries.
SYMLINK_SHLIB is the ln -sf rune for shared library symlinks.

Use these in the appropriate places in tools/libxc and tools/xenstore.

No functional change right now.  In a forthcoming patch these
variables might take on different values.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>