MINOR: ssl: add build param USE_PRIVATE_CACHE to build cache without shared memory

It removes dependencies with futex or mutex but ssl performances decrease
using nbproc > 1 because switching process force session renegotiation.

This can be useful on small systems which never intend to run in multi-process
mode.
diff --git a/Makefile b/Makefile
index 7453198..f0533a7 100644
--- a/Makefile
+++ b/Makefile
@@ -15,6 +15,7 @@
 #   USE_NETFILTER        : enable netfilter on Linux. Automatic.
 #   USE_PCRE             : enable use of libpcre for regex. Recommended.
 #   USE_POLL             : enable poll(). Automatic.
+#   USE_PRIVATE_CACHE    : disable shared memory cache of ssl sessions.
 #   USE_REGPARM          : enable regparm optimization. Recommended on x86.
 #   USE_SEPOLL           : enable speculative epoll(). Automatic.
 #   USE_STATIC_PCRE      : enable static libpcre. Recommended.
@@ -476,12 +477,16 @@
 OPTIONS_CFLAGS  += -DUSE_OPENSSL
 OPTIONS_LDFLAGS += -lssl -lcrypto
 OPTIONS_OBJS  += src/ssl_sock.o src/shctx.o
+ifneq ($(USE_PRIVATE_CACHE),)
+OPTIONS_CFLAGS  += -DUSE_PRIVATE_CACHE
+else
 ifneq ($(USE_FUTEX),)
 OPTIONS_CFLAGS  += -DUSE_SYSCALL_FUTEX
 else
 OPTIONS_LDFLAGS += -lpthread
 endif
 endif
+endif
 
 ifneq ($(USE_PCRE),)
 # PCREDIR is the directory hosting include/pcre.h and lib/libpcre.*. It is