Use libnix as a prefix for all Nix libraries

In particular "libutil" was always a problem because it collides with
Glibc's libutil.  Even if we install into $(libdir)/nix, the linker
sometimes got confused (e.g. if a program links against libstore but
not libutil, then ld would report undefined symbols in libstore
because it was looking at Glibc's libutil).

src/boost/format/Makefile.new

@@ -1,5 +1,7 @@
 LIBS += libformat
 
+libformat_NAME = libnixformat
+
 libformat_DIR := $(d)
 
 libformat_SOURCES = format_implementation.cc free_funcs.cc parsing.cc

src/libexpr/Makefile.new

@@ -1,5 +1,7 @@
 LIBS += libexpr
 
+libexpr_NAME = libnixexpr
+
 libexpr_DIR := $(d)
 
 libexpr_SOURCES = \

src/libmain/Makefile.new

@@ -1,5 +1,7 @@
 LIBS += libmain
 
+libmain_NAME = libnixmain
+
 libmain_DIR := $(d)
 
 libmain_SOURCES = shared.cc stack.cc

src/libstore/Makefile.new

@@ -1,5 +1,7 @@
 LIBS += libstore
 
+libstore_NAME = libnixstore
+
 libstore_DIR := $(d)
 
 libstore_SOURCES = \

src/libutil/Makefile.new

@@ -1,5 +1,7 @@
 LIBS += libutil
 
+libutil_NAME = libnixutil
+
 libutil_DIR := $(d)
 
 libutil_SOURCES = util.cc hash.cc serialise.cc archive.cc xml-writer.cc affinity.cc