Build against the system libperiphery instead of the c-periphery git
submodule, which the release tarball leaves empty. The headers are
found through the pkg-config Cflags, so include them by bare name.

--- a/Makefile
+++ b/Makefile
@@ -1,8 +1,9 @@
 LIB = periphery.so
 SRCS = src/lua_periphery.c src/lua_mmio.c src/lua_gpio.c src/lua_led.c src/lua_pwm.c src/lua_spi.c src/lua_i2c.c src/lua_serial.c
 
-C_PERIPHERY = c-periphery
-C_PERIPHERY_LIB = $(C_PERIPHERY)/periphery.a
+PKG_CONFIG ?= pkg-config
+C_PERIPHERY_CFLAGS = $(shell $(PKG_CONFIG) --cflags libperiphery)
+C_PERIPHERY_LIBS = $(shell $(PKG_CONFIG) --libs libperiphery)
 
 ifndef LUA
 LUA = lua
@@ -20,7 +21,7 @@
 
 MOD_CFLAGS = $(CFLAGS)
 MOD_CFLAGS += -std=c99 -pedantic -D_XOPEN_SOURCE=700
-MOD_CFLAGS += -Wall -Wextra -Wno-unused-parameter $(DEBUG) -fPIC -I. $(LUA_CFLAGS)
+MOD_CFLAGS += -Wall -Wextra -Wno-unused-parameter $(DEBUG) -fPIC -I. $(C_PERIPHERY_CFLAGS) $(LUA_CFLAGS)
 
 MOD_LDFLAGS = $(LDFLAGS)
 MOD_LDFLAGS += -shared
@@ -36,7 +37,6 @@
 
 .PHONY: clean
 clean:
-	$(MAKE) -C $(C_PERIPHERY) clean
 	rm -rf $(LIB)
 
 .PHONY: install
@@ -45,8 +45,5 @@
 
 ###########################################################################
 
-$(LIB): $(C_PERIPHERY_LIB) $(SRCS)
-	$(CC) $(MOD_CFLAGS) $(MOD_LDFLAGS) $(SRCS) $(C_PERIPHERY_LIB) -o $@
-
-$(C_PERIPHERY_LIB): $(C_PERIPHERY)/Makefile
-	$(MAKE) -C $(C_PERIPHERY)
+$(LIB): $(SRCS)
+	$(CC) $(MOD_CFLAGS) $(MOD_LDFLAGS) $(SRCS) $(C_PERIPHERY_LIBS) -o $@
--- a/src/lua_gpio.c
+++ b/src/lua_gpio.c
@@ -13,7 +13,7 @@
 #include <stdint.h>
 #include <errno.h>
 
-#include <c-periphery/src/gpio.h>
+#include <gpio.h>
 #include "lua_periphery.h"
 #include "lua_compat.h"
 
--- a/src/lua_i2c.c
+++ b/src/lua_i2c.c
@@ -14,7 +14,7 @@
 #include <stdint.h>
 #include <errno.h>
 
-#include <c-periphery/src/i2c.h>
+#include <i2c.h>
 #include "lua_periphery.h"
 #include "lua_compat.h"
 
--- a/src/lua_led.c
+++ b/src/lua_led.c
@@ -13,7 +13,7 @@
 #include <stdint.h>
 #include <errno.h>
 
-#include <c-periphery/src/led.h>
+#include <led.h>
 #include "lua_periphery.h"
 #include "lua_compat.h"
 
--- a/src/lua_mmio.c
+++ b/src/lua_mmio.c
@@ -13,7 +13,7 @@
 #include <stdint.h>
 #include <errno.h>
 
-#include <c-periphery/src/mmio.h>
+#include <mmio.h>
 #include "lua_periphery.h"
 #include "lua_compat.h"
 
--- a/src/lua_periphery.h
+++ b/src/lua_periphery.h
@@ -7,7 +7,7 @@
 #ifndef _LUA_PERIPHERY_H
 #define _LUA_PERIPHERY_H
 
-#include <c-periphery/src/version.h>
+#include <version.h>
 
 #define _STRINGIFY(x)   #x
 #define STRINGIFY(x)    _STRINGIFY(x)
--- a/src/lua_pwm.c
+++ b/src/lua_pwm.c
@@ -13,7 +13,7 @@
 #include <stdint.h>
 #include <errno.h>
 
-#include <c-periphery/src/pwm.h>
+#include <pwm.h>
 #include "lua_periphery.h"
 #include "lua_compat.h"
 
--- a/src/lua_serial.c
+++ b/src/lua_serial.c
@@ -13,7 +13,7 @@
 #include <stdint.h>
 #include <errno.h>
 
-#include <c-periphery/src/serial.h>
+#include <serial.h>
 #include "lua_periphery.h"
 #include "lua_compat.h"
 
--- a/src/lua_spi.c
+++ b/src/lua_spi.c
@@ -13,7 +13,7 @@
 #include <stdint.h>
 #include <errno.h>
 
-#include <c-periphery/src/spi.h>
+#include <spi.h>
 #include "lua_periphery.h"
 #include "lua_compat.h"