message: Makefile: enable i915-perf builds
  It remains disabled by default like for CMake.
diff: 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 \
  