tools: remove vestigial default_lib.m4 macros and adjust substitutions

tools: remove vestigial default_lib.m4 macros and adjust substitutions

LIB_PATH is no longer used, so the AX_DEFAULT_LIB macro is no longer
needed. Additionally lower case make variables are now used as
autoconf substitutions, which allows for more correct overrides at
build time.

I've checked the file layout in dist/install from the build made
before this change versus after with ./configure values of:
 1) ./configure (no flags provided)
 2) ./configure --libdir=/usr/lib/x86_64-linux-gnu (Debian style)
 3) ./configure --libdir='${exec_prefix}/lib' (late variable expansion)

Signed-off-by: Matt Wilson <msw@amazon.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc - reran autogen.sh ]
Committed-by: Ian Campbell <ian.campbell@citrix.com>