From f2ba1f6fdbcb01f3171ae44fa45ab6208d8058db Mon Sep 17 00:00:00 2001
From: Leonid Yuriev <leo@yuriev.ru>
Date: Sun, 20 Sep 2020 18:07:33 +0300
Subject: [PATCH] mdbx-make: use deferred simple variable expansion trick for
 CXXSTD.

Change-Id: I3852ff0af27c828a1ac73344330b4f5fb91e3f1f
---
 GNUmakefile | 23 ++++++++++++++++-------
 Makefile    |  4 +++-
 2 files changed, 19 insertions(+), 8 deletions(-)

diff --git a/GNUmakefile b/GNUmakefile
index f0951e1b..80ac33ac 100644
--- a/GNUmakefile
+++ b/GNUmakefile
@@ -1,11 +1,19 @@
-# This makefile is for GNU Make, and nowadays provided
+# This makefile is for GNU Make 3.80 or above, and nowadays provided
 # just for compatibility and preservation of traditions.
-# Please use CMake in case of any difficulties or problems.
 #
-# Preprocessor macros (for MDBX_OPTIONS) of interest...
+# Please use CMake in case of any difficulties or
+# problems with this old-school's magic.
+#
+################################################################################
+#
+# Preprocessor macros (for MDBX_OPTIONS) of interest.
+#
 # Note that the defaults should already be correct for most platforms;
 # you should not need to change any of these. Read their descriptions
-# in README and source code if you do. There may be other macros of interest.
+# in README and source code (see src/options.h) if you do.
+# There may be other macros of interest.
+#
+
 SHELL   := env bash
 
 # install sandbox
@@ -25,8 +33,9 @@ MDBX_OPTIONS ?= -DNDEBUG=1
 CFLAGS  ?= -O2 -g -Wall -Werror -Wextra -Wpedantic -ffunction-sections -fPIC -fvisibility=hidden -std=gnu11 -pthread -Wno-error=attributes $(CFLAGS_EXTRA)
 # -Wno-tautological-compare
 CXX     ?= g++
-CXXSTD  ?= $(shell PROBE=$$([ -f mdbx.c++ ] && echo mdbx.c++ || echo src/mdbx.c++); for std in gnu++20 c++20 gnu++2a c++2a gnu++17 c++17 gnu++14 c++14 gnu+11 c++11; do $(CXX) -std=$${std} -c $${PROBE} -o /dev/null 2>/dev/null >/dev/null && echo "-std=$${std}" && exit; done)
-CXXFLAGS:= $(CXXSTD) $(filter-out -std=gnu11,$(CFLAGS))
+# choicing C++ standard with deferred simple variable expansion trick
+CXXSTD  ?= $(eval CXXSTD := $$(shell PROBE=$$$$([ -f mdbx.c++ ] && echo mdbx.c++ || echo src/mdbx.c++); for std in gnu++20 c++20 gnu++2a c++2a gnu++17 c++17 gnu++14 c++14 gnu+11 c++11; do $(CXX) -std=$$$${std} -c $$$${PROBE} -o /dev/null 2>/dev/null >/dev/null && echo "-std=$$$${std}" && exit; done))$(CXXSTD)
+CXXFLAGS = $(CXXSTD) $(filter-out -std=gnu11,$(CFLAGS))
 
 # HINT: Try append '--no-as-needed,-lrt' for ability to built with modern glibc, but then run with the old.
 LIBS    ?= $(shell uname | grep -qi SunOS && echo "-lkstat") $(shell uname | grep -qi -e Darwin -e OpenBSD || echo "-lrt") $(shell uname | grep -qi Windows && echo "-lntdll")
@@ -209,7 +218,7 @@ build-test: all mdbx_example mdbx_test
 
 define test-rule
 $(patsubst %.cc,%.o,$(1)): $(1) $(TEST_INC) mdbx.h $(lastword $(MAKEFILE_LIST))
-	$(CXX) $(CXXFLAGS) $(MDBX_OPTIONS) -c $(1) -o $$@
+	$$(CXX) $$(CXXFLAGS) $$(MDBX_OPTIONS) -c $(1) -o $$@
 
 endef
 $(foreach file,$(TEST_SRC),$(eval $(call test-rule,$(file))))
diff --git a/Makefile b/Makefile
index 50ba4443..bb4aa053 100644
--- a/Makefile
+++ b/Makefile
@@ -1,5 +1,7 @@
+# This is thunk-Makefile for calling GNU Make 3.80 or above
+
 all bench bench-quartet build-test check clean clean-bench cross-gcc cross-qemu dist doxygen gcc-analyzer install mdbx memcheck reformat release-assets strip test test-asan test-fault test-leak test-singleprocess test-ubsan test-valgrind tools:
 	@CC=$(CC) \
 	CXX=`if test -n "$(CXX)" && which "$(CXX)" > /dev/null; then echo "$(CXX)"; elif test -n "$(CCC)" && which "$(CCC)" > /dev/null; then echo "$(CCC)"; else echo "c++"; fi` \
-	`which gmake || which gnumake || echo 'echo "GNU Make is required"; exit 2;'` \
+	`which gmake || which gnumake || echo 'echo "GNU Make 3.80 or above is required"; exit 2;'` \
 		$(MAKEFLAGS) -f GNUmakefile $@