Request: make USE_SSE=1 the default

I build with the
```
PORTABLE=1 make static_lib
```
the make_config.mk showed no -DHAVE_SSE42 will be used which will disable the fast crc32
```
CC=cc
CXX=g++
PLATFORM=OS_LINUX
PLATFORM_LDFLAGS= -lpthread -lrt -lsnappy -lz -lbz2
JAVA_LDFLAGS= -lpthread -lrt -lsnappy -lz -lbz2
JAVA_STATIC_LDFLAGS= -lpthread -lrt
VALGRIND_VER=
PLATFORM_CCFLAGS= -DROCKSDB_PLATFORM_POSIX -DROCKSDB_LIB_IO_POSIX -DOS_LINUX -fno-builtin-memcmp -DROCKSDB_FALLOCATE_PRESENT -DSNAPPY -DZLIB -DBZIP2 -DROCKSDB_MALLOC_USABLE_SIZE -DROCKSDB_PTHREAD_ADAPTIVE_MUTEX -DROCKSDB_BACKTRACE -DROCKSDB_RANGESYNC_PRESENT -DROCKSDB_SCHED_GETCPU_PRESENT -DROCKSDB_SUPPORT_THREAD_LOCAL
PLATFORM_CXXFLAGS=-std=c++11 -DROCKSDB_PLATFORM_POSIX -DROCKSDB_LIB_IO_POSIX -DOS_LINUX -fno-builtin-memcmp -DROCKSDB_FALLOCATE_PRESENT -DSNAPPY -DZLIB -DBZIP2 -DROCKSDB_MALLOC_USABLE_SIZE -DROCKSDB_PTHREAD_ADAPTIVE_MUTEX -DROCKSDB_BACKTRACE -DROCKSDB_RANGESYNC_PRESENT -DROCKSDB_SCHED_GETCPU_PRESENT -DROCKSDB_SUPPORT_THREAD_LOCAL
PLATFORM_SHARED_CFLAGS=-fPIC
```

While I build with the default `make static_lib` without using the PORTABLE, the content in the make_config.mk showed
```
CC=cc
CXX=g++
PLATFORM=OS_LINUX
PLATFORM_LDFLAGS= -lpthread -lrt -lsnappy -lz -lbz2
JAVA_LDFLAGS= -lpthread -lrt -lsnappy -lz -lbz2
JAVA_STATIC_LDFLAGS= -lpthread -lrt
VALGRIND_VER=
PLATFORM_CCFLAGS= -DROCKSDB_PLATFORM_POSIX -DROCKSDB_LIB_IO_POSIX -DOS_LINUX -fno-builtin-memcmp -DROCKSDB_FALLOCATE_PRESENT -DSNAPPY -DZLIB -DBZIP2 -DROCKSDB_MALLOC_USABLE_SIZE -DROCKSDB_PTHREAD_ADAPTIVE_MUTEX -DROCKSDB_BACKTRACE -DROCKSDB_RANGESYNC_PRESENT -DROCKSDB_SCHED_GETCPU_PRESENT -march=native -DHAVE_SSE42 -DHAVE_PCLMUL -DROCKSDB_SUPPORT_THREAD_LOCAL
PLATFORM_CXXFLAGS=-std=c++11 -DROCKSDB_PLATFORM_POSIX -DROCKSDB_LIB_IO_POSIX -DOS_LINUX -fno-builtin-memcmp -DROCKSDB_FALLOCATE_PRESENT -DSNAPPY -DZLIB -DBZIP2 -DROCKSDB_MALLOC_USABLE_SIZE -DROCKSDB_PTHREAD_ADAPTIVE_MUTEX -DROCKSDB_BACKTRACE -DROCKSDB_RANGESYNC_PRESENT -DROCKSDB_SCHED_GETCPU_PRESENT -march=native -DHAVE_SSE42 -DHAVE_PCLMUL -DROCKSDB_SUPPORT_THREAD_LOCAL
PLATFORM_SHARED_CFLAGS=-fPIC
```

I can force using the sse by `USE_SSE=1 PORTABLE=1 make static_lib`.

So, is it expected for a portable build?