Add the link path for the externals-clasp libraries to the built Clasp executables.

Only tested on Ubuntu 14.04 / Linux

But without adding the link path:
mband@<host>:~$ ldd ~/local/clasp/bin/clasp_boehm_o 
	linux-vdso.so.1 =>  (0x00007fff29ffe000)
	libgmpxx.so.4 => not found
	libgmp.so.10 => /usr/lib/x86_64-linux-gnu/libgmp.so.10 (0x00007feca52ca000)
	libncurses.so.5 => /lib/x86_64-linux-gnu/libncurses.so.5 (0x00007feca50a6000)
	libtinfo.so.5 => /lib/x86_64-linux-gnu/libtinfo.so.5 (0x00007feca4e7d000)
	libreadline.so.6 => /lib/x86_64-linux-gnu/libreadline.so.6 (0x00007feca4c37000)
	libz.so.1 => /lib/x86_64-linux-gnu/libz.so.1 (0x00007feca4a1d000)
	libexpat.so.1 => /lib/x86_64-linux-gnu/libexpat.so.1 (0x00007feca47f3000)
	librt.so.1 => /lib/x86_64-linux-gnu/librt.so.1 (0x00007feca45eb000)
	libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007feca43cc000)
	libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007feca41c8000)
	libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007feca3ec4000)
	libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007feca3bbd000)
	libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007feca39a7000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007feca35e1000)
	/lib64/ld-linux-x86-64.so.2 (0x00007feca556f000)

And with this patch:
mband@<host>:~$ ldd ~/local/clasp/bin/clasp_boehm_o 
	linux-vdso.so.1 =>  (0x00007fff1b1e9000)
	libgmpxx.so.4 => /home/mband/local/externals-clasp/common/lib/libgmpxx.so.4 (0x00007fcdabede000)
	libgmp.so.10 => /home/mband/local/externals-clasp/common/lib/libgmp.so.10 (0x00007fcdabc67000)
	libncurses.so.5 => /lib/x86_64-linux-gnu/libncurses.so.5 (0x00007fcdaba15000)
	libtinfo.so.5 => /lib/x86_64-linux-gnu/libtinfo.so.5 (0x00007fcdab7ec000)
	libreadline.so.6 => /home/mband/local/externals-clasp/common/lib/libreadline.so.6 (0x00007fcdab5a8000)
	libz.so.1 => /home/mband/local/externals-clasp/common/lib/libz.so.1 (0x00007fcdab38e000)
	libexpat.so.1 => /home/mband/local/externals-clasp/common/lib/libexpat.so.1 (0x00007fcdab164000)
	librt.so.1 => /lib/x86_64-linux-gnu/librt.so.1 (0x00007fcdaaf5b000)
	libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007fcdaad3d000)
	libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007fcdaab39000)
	libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007fcdaa834000)
	libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007fcdaa52e000)
	libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007fcdaa318000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fcda9f51000)
	/lib64/ld-linux-x86-64.so.2 (0x00007fcdac0e6000)