Fix Makefile handling of user CXXFLAGS & also add a DEBUG flag (#990)

The way `CXXFLAGS` and other flags were being set meant that users had
to add the default values (`-std=c++17 -fopenmp -O3` etc) if they set
the flags at all, or else lose the defaults. This was error-prone and
suboptimal. The change here makes it so that the user values are
appended at the end of the defaults, so that they can override values.

This PR also adds a DEBUG flag that switches between using `-g -O0` and
the regular `-O3` option. Developers can invoke it using, e.g., `make
DEBUG=1`.

Author: mhucka

Makefile

@@ -16,11 +16,12 @@
 EIGEN_COMMIT = "b66188b5dfd147265bfa9ec47595ca0db72d21f5"
 EIGEN_URL = "https://gitlab.com/libeigen/eigen/-/archive/"
 
-# Default build targets. Additional may be added conditionally below.
+# Default build targets. Additional ones are added conditionally below.
 TARGETS = qsim
 TESTS = run-cxx-tests
 
 # By default, we also build the pybind11-based Python interface.
+# Can be overriden via env variables or command-line flags
 PYBIND11 ?= true
 
 ifeq ($(PYBIND11), true)
@@ -31,14 +32,36 @@ endif
 # Default options for Pytest (only used if the pybind interface is built).
 PYTESTFLAGS ?= -v
 
-# Default C++ compilers and compiler flags. Can be overriden via env variables.
+# Default compilers and compiler flags.
+# Can be overriden via env variables or command-line flags.
 CXX ?= g++
 NVCC ?= nvcc
 HIPCC ?= hipcc
 
-CXXFLAGS ?= -O3 -std=c++17 -fopenmp -flto=auto
-NVCCFLAGS ?= -O3 --std c++17 -Wno-deprecated-gpu-targets
-HIPCCFLAGS ?= -O3
+BASE_CXXFLAGS := -std=c++17 -fopenmp
+BASE_NVCCFLAGS := -std c++17 -Wno-deprecated-gpu-targets
+BASE_HIPCCFLAGS :=
+
+CXXFLAGS := $(BASE_CXXFLAGS) $(CXXFLAGS)
+NVCCFLAGS := $(BASE_NVCCFLAGS) $(NVCCFLAGS)
+HIPCCFLAGS := $(BASE_HIPCCFLAGS) $(HIPCCFLAGS)
+
+LTO_FLAGS := -flto=auto
+USING_CLANG := $(shell $(CXX) --version | grep -isq clang && echo "true")
+ifeq ($(USING_CLANG),"true")
+	LTO_FLAGS := -flto
+endif
+
+ifdef DEBUG
+    DEBUG_FLAGS := -g -O0
+    CXXFLAGS += $(DEBUG_FLAGS)
+    NVCCFLAGS += $(DEBUG_FLAGS)
+    HIPCCFLAGS += $(DEBUG_FLAGS)
+else
+    CXXFLAGS += -O3 $(LTO_FLAGS)
+    NVCCFLAGS += -O3
+    HIPCCFLAGS += -O3
+endif
 
 # For compatibility with CMake, if $CUDAARCHS is set, use it to set the
 # architecture options to nvcc. Otherwise, default to the "native" option,