Allow GNU/Linux maintainers to select configure options without

patching of Makefile.

Signed-off-by: Vitaly Zaitsev <vitaly@easycoding.org>

src/Makefile

@@ -3,16 +3,16 @@
 ## License.....: MIT
 ##
 
-SHARED                  := 0
+SHARED                  ?= 0
 DEBUG                   := 0
 PRODUCTION              := 0
 PRODUCTION_VERSION      := v6.0.0
-ENABLE_CUBIN            := 1
+ENABLE_CUBIN            ?= 1
-ENABLE_BRAIN            := 1
+ENABLE_BRAIN            ?= 1
-USE_SYSTEM_LZMA         := 0
+USE_SYSTEM_LZMA         ?= 0
-USE_SYSTEM_ZLIB         := 0
+USE_SYSTEM_ZLIB         ?= 0
-USE_SYSTEM_OPENCL       := 0
+USE_SYSTEM_OPENCL       ?= 0
-USE_SYSTEM_XXHASH       := 0
+USE_SYSTEM_XXHASH       ?= 0
 
 ##
 ## Detect Operating System