DLPATH and make

[videlec]

Similarly to flint issue #447 arb fails to build in Sage when make is compiled with libguile support (that happens to be the case on archlinux). As a consequence, modifying LD_LIBRARY_PATH causes build failure (see [https://trac.sagemath.org/ticket/24575 upstream issue]). As in flint, the culprit are the lines 1-5 of Makefile.in.

ifdef $(DLPATH)
  $(DLPATH):=$($(DLPATH)):$(DLPATH_ADD)
else
  $(DLPATH):=$(DLPATH_ADD)
endif

Removing them allow to build arb without troubles.

Note that we also have an alternative workaround in Sage using LD_PRELOAD (see the branch on upstream ticket).