Makefile: make portable, support static linking

We can happily stick to POSIX and the usual variable names,
with the following exceptions:
 - pkg-config(1) is used to find libraries and compile flags
 - the new "static" target uses non-standard compiler
   and linker flags, though it's not the default target

sdn is predestined for wild distribution,
even the dynamically linked libc is rather suboptimal.

Makefile

@@ -1,12 +1,19 @@
+.POSIX:
 SHELL = /bin/sh
-CXXFLAGS = -g -std=c++14 -Wall -Wextra -pedantic -static-libstdc++
-
-all: sdn
-%: %.cpp CMakeLists.txt
-	$(CXX) $(CXXFLAGS) $< -o $@ `pkg-config --libs --cflags ncursesw` -lacl \
-	`sed -ne 's/^project (\([^ )]*\).*/-DPROJECT_NAME="\1"/p' \
+CXXFLAGS = -g -std=c++14 -Wall -Wextra -pedantic
+CPPFLAGS = `sed -ne 's/^project (\([^ )]*\).*/-DPROJECT_NAME="\1"/p' \
 	-e 's/^set (version \([^ )]*\).*/-DPROJECT_VERSION="\1"/p' CMakeLists.txt`
+
+sdn: sdn.cpp CMakeLists.txt
+	$(CXX) $(CXXFLAGS) $(CPPFLAGS) $< -o sdn \
+	-lacl `pkg-config --libs --cflags ncursesw`
+static: sdn.cpp CMakeLists.txt
+	$(CXX) $(CXXFLAGS) $(CPPFLAGS) $< -o sdn \
+	-static-libstdc++ \
+	-Wl,--start-group,-Bstatic \
+	-lacl `pkg-config --static --libs --cflags ncursesw` \
+	-Wl,--end-group,-Bdynamic
 clean:
 	rm -f sdn
 
-.PHONY: all clean
+.PHONY: static clean