More testing Make configure.R::setRcppTskitLibAndFlags() portable across Unix/Linux/macOS/Windows platforms

Fixes #19

Author: gregorgorjanc

RcppTskit/src/Makevars.in

@@ -46,7 +46,7 @@ PKG_CFLAGS = -DNDEBUG # to remove calls to assert() as per the R extensions manu
 
 # Explicit compile rule for tskit C files
 tskit/%.o: tskit/%.c
-	$(CC) $(PKG_CPPFLAGS) $(PKG_CFLAGS) $(CFLAGS) -c $< -o $@
+	$(CC) $(PKG_CPPFLAGS) $(PKG_CFLAGS) $(CFLAGS) $(CPICFLAGS) -c $< -o $@
 
 # Linking
 PKG_LIBS = @RCPPTSKIT_LIB@