Add a make target for shared library

Summary:
Previous we made `make release` also compile shared library. However it takes a long time to complete.

To make our development process more efficient. I added a new make target shared_lib.

User can of course run `make <library_name>` for direct compilation. However the <library_name> changed under certain condition. Thus we need `make shared_lib` to get rid of the memorization from users' side.

Test Plan: make shared_lib

Reviewers: igor, sdong, haobo, dhruba

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15309

Makefile

@@ -135,13 +135,12 @@ endif  # PLATFORM_SHARED_EXT
 all: $(LIBRARY) $(PROGRAMS)
 
 .PHONY: blackbox_crash_test check clean coverage crash_test ldb_tests \
-	release tags valgrind_check whitebox_crash_test format
+	release tags valgrind_check whitebox_crash_test format shared_lib
 
 # Will also generate shared libraries. 
 release:
 	$(MAKE) clean
 	OPT=-DNDEBUG $(MAKE) all -j32
-	OPT=-DNDEBUG $(MAKE) $(SHARED) -j32
 
 coverage:
 	$(MAKE) clean
@@ -201,6 +200,8 @@ tags:
 format:
 	build_tools/format-diff.sh
 
+shared_lib: $(SHARED)
+
 # ---------------------------------------------------------------------------
 # 	Unit tests and tools
 # ---------------------------------------------------------------------------