add basic example for compiling upkr to a c library

c_library/Makefile

@@ -0,0 +1,8 @@
+upkr: upkr.c upkr.h target/release/libupkr.a
+	gcc -O2 -Ltarget/release -o upkr upkr.c -lupkr -lm
+	strip upkr
+
+target/release/libupkr.a: cargo
+	cargo build --release
+
+.PHONY: cargo