use LANG_VERSION to switch from C++14 (default) to C++17, if necessary

Makefile.conf

@@ -13,7 +13,7 @@ PREFIX?=$(HOME)
 ENGINE_LIB_PATH=$(PREFIX)/lib
 ENGINE_INC_PATH=$(PREFIX)/include
 
-CXXFLAGS+=-std=c++14 -fPIC
+CXXFLAGS+=-fPIC
 
 # Build target
 BUILD_FOR:=$(shell uname)
@@ -34,6 +34,13 @@ else
     CXXFLAGS+=-DNDEBUG=1 -O3
 endif
 
+ifdef $(LANG_VERSION)
+CXXFLAGS+=-std=$(LANG_VERSION)
+else
+CXXFLAGS+=-std=c++14
+endif
+
+
 # Add -L Prefixes to LIB/INC paths,
 # if not already explicitly set in local.conf
 ifndef ENGINE_LIB