Makefile: enable i915-perf builds

It remains disabled by default like for CMake.

Makefile

@@ -16,6 +16,7 @@
 NAME = gpuvis
 
 USE_GTK3 ?= 1
+USE_I915_PERF ?= 0
 CFG ?= release
 ifeq ($(CFG), debug)
     ASAN ?= 1
@@ -35,6 +36,12 @@ ifeq ($(USE_GTK3), 1)
 GTK3FLAGS=$(shell pkg-config --cflags gtk+-3.0) -DUSE_GTK3
 endif
 
+ifeq ($(USE_I915_PERF), 1)
+I915_PERF_CFLAGS=$(shell pkg-config --cflags i915-perf) -DUSE_I915_PERF
+I915_PERF_LIBS=$(shell pkg-config --libs i915-perf)
+endif
+
+
 WARNINGS = -Wall -Wextra -Wpedantic -Wmissing-include-dirs -Wformat=2 -Wshadow -Wno-unused-parameter -Wno-missing-field-initializers
 ifneq ($(COMPILER),clang)
   # https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html
@@ -44,11 +51,11 @@ endif
 # Investigate: Improving C++ Builds with Split DWARF
 #  http://www.productive-cpp.com/improving-cpp-builds-with-split-dwarf/
 
-CFLAGS = $(WARNINGS) -march=native -fno-exceptions -gdwarf-4 -g2 -ggnu-pubnames -gsplit-dwarf $(SDL2FLAGS) $(GTK3FLAGS) -I/usr/include/freetype2
+CFLAGS = $(WARNINGS) -march=native -fno-exceptions -gdwarf-4 -g2 -ggnu-pubnames -gsplit-dwarf $(SDL2FLAGS) $(GTK3FLAGS) $(I915_PERF_CFLAGS) -I/usr/include/freetype2
 CFLAGS += -DUSE_FREETYPE -D_LARGEFILE64_SOURCE=1 -D_FILE_OFFSET_BITS=64
 CXXFLAGS = -fno-rtti -Woverloaded-virtual -Wno-class-memaccess
 LDFLAGS = -march=native -gdwarf-4 -g2 -Wl,--build-id=sha1
-LIBS = -Wl,--no-as-needed -lm -ldl -lpthread -lfreetype -lstdc++ $(SDL2LIBS)
+LIBS = -Wl,--no-as-needed -lm -ldl -lpthread -lfreetype -lstdc++ $(SDL2LIBS) $(I915_PERF_LIBS)
 
 ifneq ("$(wildcard /usr/bin/ld.gold)","")
   $(info Using gold linker...)
@@ -78,6 +85,7 @@ CFILES = \
 	src/imgui/imgui_demo.cpp \
 	src/imgui/imgui_draw.cpp \
 	src/GL/gl3w.c \
+	src/i915-perf/i915-perf-read.cpp \
 	src/trace-cmd/event-parse.c \
 	src/trace-cmd/trace-seq.c \
 	src/trace-cmd/kbuffer-parse.c \