Summary:
Any time one would modify a dependent of any *test*.cc file,
"make" would fail to rebuild the affected test binaries,
e.g., db_test.  That was due to the fact that we deliberately
excluded those test-related files from the definition of SOURCES
and only $(SOURCES) was used to create the automatically-generated
.d dependency files.  The fix is to generate a .d file for every
source file.
* src.mk: New file.  Defines LIB_SOURCES, MOCK_SOURCES
and TEST_BENCH_SOURCES.
* Makefile: Include src.mk.
Reflect s/SOURCES/LIB_SOURCES/ renaming.
* build_tools/build_detect_platform: Remove the code
that was used to generate SOURCES= and MOCK_SOURCES=
definitions in make_config.mk. Those lists of files
are now hard-coded in src.mk. Hard-coding this list of
sources is desirable, because without that, one risks
including stray .cc files in a build.  Not reproducible.

Test Plan:
Touch a file used by db_test's dependent .o files and ensure that
they are all recompiled.  Before, none would be:

  $ touch db/db_impl.h && make db_test
    CC       db/db_test.o
    CC       db/column_family.o
    CC       db/db_filesnapshot.o
    CC       db/db_impl.o
    CC       db/db_impl_debug.o
    CC       db/db_impl_readonly.o
    CC       db/forward_iterator.o
    CC       db/internal_stats.o
    CC       db/managed_iterator.o
    CC       db/repair.o
    CC       db/write_batch.o
    CC       utilities/compacted_db/compacted_db_impl.o
    CC       utilities/ttl/db_ttl_impl.o
    CC       util/ldb_cmd.o
    CC       util/ldb_tool.o
    CC       util/sst_dump_tool.o
    CC       util/xfunc.o
    CCLD     db_test

Reviewers: ljin, igor.sugak, igor, rven, sdong

Reviewed By: sdong

Subscribers: yhchiang, adamretter, fyrz, dhruba

Differential Revision: https://reviews.facebook.net/D33849