[build] make all is default; Bump to LibZipSharp/1b217dab (#677)

Commit 8ebb9cb inadvertently made `make prepare` the default target
when running on Linux, instead of `make all`, breaking semantics:

	$ make
	# pre-8ebb9cb2, invoked `make all` everywhere.
	# Invokes `make prepare` on Linux in 8ebb9cb, but
	# `make all` on macOS. Confusion!

Revert the default `make` target to `make all`.

Bump to LibZipSharp/master/1b217dab to increase reliability of
`libZipSharp.dll.config` creation on Linux if/when
`ldconfig -p | grep libzip` returns more than one shared library.

Makefile

@@ -19,18 +19,18 @@ ifneq ($(MONO_OPTIONS),)
 export MONO_OPTIONS
 endif
 
-ifeq ($(OS),Linux)
-export LINUX_DISTRO         := $(shell lsb_release -i -s || true)
-export LINUX_DISTRO_RELEASE := $(shell lsb_release -r -s || true)
-prepare:: linux-prepare
-endif # $(OS)=Linux
-
 include build-tools/scripts/msbuild.mk
 all::
 	$(MSBUILD) $(MSBUILD_FLAGS) $(SOLUTION)
 
 all-tests::
 	MSBUILD="$(MSBUILD)" tools/scripts/xabuild $(MSBUILD_FLAGS) Xamarin.Android-Tests.sln
+
+ifeq ($(OS),Linux)
+export LINUX_DISTRO         := $(shell lsb_release -i -s || true)
+export LINUX_DISTRO_RELEASE := $(shell lsb_release -r -s || true)
+prepare:: linux-prepare
+endif # $(OS)=Linux
 
 prepare:: prepare-msbuild