[contrib/pzstd] Detect and Select Maximum Available C++ Standard

Rather than remove the flag entirely, as proposed in #3499, this commit uses
the newest C++ standard the compiler supports. This retains the selection of
using only standardized features (excluding GNU extensions) and keeps the
recency requirements of the codebase explicit.

Tested with various versions of `g++` and `clang++`.

diff --git a/contrib/pzstd/Makefile b/contrib/pzstd/Makefile
index 830053cd70b934aa9618905cf1d18e70280f30f1..423be59d283caa1d25745117d9ce527109aa4ffa 100644 (file)
--- a/contrib/pzstd/Makefile
+++ b/contrib/pzstd/Makefile
@@ -37,10 +37,29 @@ CFLAGS   += -Wno-deprecated-declarations
 PZSTD_INC  = -I$(ZSTDDIR) -I$(ZSTDDIR)/common -I$(PROGDIR) -I.
 GTEST_INC  = -isystem googletest/googletest/include
 
+# Select newest C++ standard available, minimum C++11
+ifeq ($(shell echo | $(CXX) -x c++ -Werror -std=c++23 -c - -o /dev/null 2>/dev/null && echo 1 || echo 0),1)
+PZSTD_CXX_STD := -std=c++23
+else
+ifeq ($(shell echo | $(CXX) -x c++ -Werror -std=c++20 -c - -o /dev/null 2>/dev/null && echo 1 || echo 0),1)
+PZSTD_CXX_STD := -std=c++20
+else
+ifeq ($(shell echo | $(CXX) -x c++ -Werror -std=c++17 -c - -o /dev/null 2>/dev/null && echo 1 || echo 0),1)
+PZSTD_CXX_STD := -std=c++17
+else
+ifeq ($(shell echo | $(CXX) -x c++ -Werror -std=c++14 -c - -o /dev/null 2>/dev/null && echo 1 || echo 0),1)
+PZSTD_CXX_STD := -std=c++14
+else
+PZSTD_CXX_STD := -std=c++11
+endif
+endif
+endif
+endif
+
 PZSTD_CPPFLAGS  = $(PZSTD_INC)
 PZSTD_CCXXFLAGS =
 PZSTD_CFLAGS    = $(PZSTD_CCXXFLAGS)
-PZSTD_CXXFLAGS  = $(PZSTD_CCXXFLAGS) -std=c++11
+PZSTD_CXXFLAGS  = $(PZSTD_CCXXFLAGS) $(PZSTD_CXX_STD)
 PZSTD_LDFLAGS   =
 EXTRA_FLAGS     =
 ALL_CFLAGS      = $(EXTRA_FLAGS) $(CPPFLAGS) $(PZSTD_CPPFLAGS) $(CFLAGS)   $(PZSTD_CFLAGS)