message: Add clang detection to makefile and clean up gcc version code
diff: NAME = gpuvis
  -CFG ?= release
  USE_GTK3 ?= 1
  -
  -# Use gcc-6 / gcc-5 if it exists and CC is the default cc
  -ifeq ($(CC),cc)
  - ifneq ("$(wildcard /usr/bin/gcc-6)","")
  - ifeq ($(CFG), debug)
  - ASAN ?= 1
  - endif
  - CC = gcc-6
  - CXX = g++-6
  - WSHADOW = -Wshadow
  - else ifneq ("$(wildcard /usr/bin/gcc-5)","")
  +CFG ?= release
  ifeq ($(CFG), debug)
  ASAN ?= 1
  endif
  - CC = gcc-5
  - CXX = g++-5
  - WSHADOW = -Wshadow
  - endif
  -endif
  LD = $(CC)
  RM = rm -f
  MKDIR = mkdir -p
  VERBOSE ?= 0
  +COMPILER = $(shell $(CC) -v 2>&1 | grep -q "clang version" && echo clang || echo gcc)
  +
  SDL2FLAGS=$(shell sdl2-config --cflags)
  SDL2LIBS=$(shell sdl2-config --static-libs)
  @@ -50,7 +36,11 @@ GTK3FLAGS=$(shell pkg-config --cflags gtk+-3.0) -DUSE_GTK3
  GTK3LIBS=$(shell pkg-config --libs gtk+-3.0)
  endif
  -WARNINGS = -Wall -Wextra -Wpedantic -Wmissing-include-dirs -Wformat=2 -Wsuggest-attribute=format $(WSHADOW) -Wno-unused-parameter -Wno-missing-field-initializers -DUSE_FREETYPE
  +WARNINGS = -Wall -Wextra -Wpedantic -Wmissing-include-dirs -Wformat=2 -Wshadow -Wno-unused-parameter -Wno-missing-field-initializers -DUSE_FREETYPE
  +ifneq ($(COMPILER),clang)
  + WARNINGS += -Wsuggest-attribute=format
  +endif
  +
  CFLAGS = $(WARNINGS) -march=native -fno-exceptions -gdwarf-4 -g2 $(SDL2FLAGS) $(GTK3FLAGS) -I/usr/include/freetype2
  CXXFLAGS = -fno-rtti -Woverloaded-virtual
  LDFLAGS = -march=native -gdwarf-4 -g2 -Wl,--build-id=sha1
  