Add Space-Optimized Helper Variable to Lib Makefile

This diff reorganizes the `lib/Makefile` to extract various settings that a
user would normally invoke together (supposing that they were aware of them)
if they were trying to build the smallest `libzstd` possible. It collects
these settings under a master setting `ZSTD_LIB_MIN_SIZE`.

Also document this new option.

lib/Makefile

@@ -24,7 +24,6 @@ CPPFLAGS+= -I. -I./common -DXXH_NAMESPACE=ZSTD_
 ifeq ($(OS),Windows_NT)   # MinGW assumed
 CPPFLAGS   += -D__USE_MINGW_ANSI_STDIO   # compatibility with %zu formatting
 endif
-CFLAGS  ?= -O3
 DEBUGFLAGS= -Wall -Wextra -Wcast-qual -Wcast-align -Wshadow \
             -Wstrict-aliasing=1 -Wswitch-enum -Wdeclaration-after-statement \
             -Wstrict-prototypes -Wundef -Wpointer-arith \
@@ -51,18 +50,46 @@ ifeq ($(findstring GCC,$(CCVER)),GCC)
 decompress/zstd_decompress_block.o :	CFLAGS+=-fno-tree-vectorize
 endif
 
-ZSTD_LEGACY_SUPPORT ?= 5
+# This is a helper variable that configures a bunch of other variables to new,
+# space-optimized defaults.
+ZSTD_LIB_MINIFY ?= 0
+ifneq ($(ZSTD_LIB_MINIFY), 0)
+	HAVE_CC_OZ ?= $(shell echo "" | $(CC) -Oz -x c -c - -o /dev/null 2> /dev/null && echo 1 || echo 0)
+	ZSTD_LEGACY_SUPPORT ?= 0
+	ZSTD_LIB_DEPRECATED ?= 0
+	HUF_FORCE_DECOMPRESS_X1 ?= 1
+	ZSTD_FORCE_DECOMPRESS_SHORT ?= 1
+	ZSTD_NO_INLINE ?= 1
+	ZSTD_STRIP_ERROR_STRINGS ?= 1
+	ifneq ($(HAVE_CC_OZ), 0)
+		# Some compilers (clang) support an even more space-optimized setting.
+		CFLAGS += -Oz
+	else
+		CFLAGS += -Os
+	endif
+	CFLAGS += -fno-stack-protector -fomit-frame-pointer -fno-ident \
+	          -DDYNAMIC_BMI2=0 -DNDEBUG
+else
+	CFLAGS += -O3
+endif
+
+# Modules
 ZSTD_LIB_COMPRESSION ?= 1
 ZSTD_LIB_DECOMPRESSION ?= 1
 ZSTD_LIB_DICTBUILDER ?= 1
 ZSTD_LIB_DEPRECATED ?= 1
+
+# Legacy support
+ZSTD_LEGACY_SUPPORT ?= 5
+ZSTD_LEGACY_MULTITHREADED_API ?= 0
+
+# Build size optimizations
 HUF_FORCE_DECOMPRESS_X1 ?= 0
 HUF_FORCE_DECOMPRESS_X2 ?= 0
 ZSTD_FORCE_DECOMPRESS_SHORT ?= 0
 ZSTD_FORCE_DECOMPRESS_LONG ?= 0
 ZSTD_NO_INLINE ?= 0
 ZSTD_STRIP_ERROR_STRINGS ?= 0
-ZSTD_LEGACY_MULTITHREADED_API ?= 0
 
 ifeq ($(ZSTD_LIB_COMPRESSION), 0)
 	ZSTD_LIB_DICTBUILDER = 0