build CXXFLAGS not appropriately manipulated when user CXXFLAGS are present

[ajakk]

I've added a small patch to my tree to help demonstrate the issue:

$ gd
diff --git a/build/flavour-unittest.mk b/build/flavour-unittest.mk
index c6f479876..71b9f657c 100644
--- a/build/flavour-unittest.mk
+++ b/build/flavour-unittest.mk
@@ -2,7 +2,9 @@
 # Build executable that runs unit tests.

 # Debug flags.
+$(info B CXXFLAGS: $(CXXFLAGS))
 CXXFLAGS+=-O3 -g -DUNITTEST -IContrib/catch2 -fsanitize=address
+$(info C CXXFLAGS: $(CXXFLAGS))

 # Strip executable?
 OPENMSX_STRIP:=false
diff --git a/build/main.mk b/build/main.mk
index 3b7ba5889..d6c9cd02d 100644
--- a/build/main.mk
+++ b/build/main.mk
@@ -187,7 +187,9 @@ TARGET_FLAGS+=$(shell $(PYTHON) build/cpu2flags.py $(OPENMSX_TARGET_CPU))
 # ========

 # Load flavour specific settings.
+$(info A CXXFLAGS: $(CXXFLAGS))
 include build/flavour-$(OPENMSX_FLAVOUR).mk
+$(info D CXXFLAGS: $(CXXFLAGS))

 UNITTEST?=false

@@ -442,6 +444,7 @@ config:
        @echo "  Platform: $(PLATFORM)"
        @echo "  Flavour:  $(OPENMSX_FLAVOUR)"
        @echo "  Compiler: $(CXX)"
+       @echo "  CXXFLAGS: $(CXXFLAGS)"
        @echo "  Subset:   $(if $(OPENMSX_SUBSET),$(OPENMSX_SUBSET)*,full build)"
 endif

A raw unittest invocation yields:

~/git/openMSX $ make OPENMSX_FLAVOUR=unittest
Using Python: python3
A CXXFLAGS:
B CXXFLAGS:
C CXXFLAGS: -O3 -g -DUNITTEST -IContrib/catch2 -fsanitize=address
D CXXFLAGS: -O3 -g -DUNITTEST -IContrib/catch2 -fsanitize=address
Build configuration:
  Platform: x86_64-linux
  Flavour:  unittest
  Compiler: x86_64-pc-linux-gnu-g++
  CXXFLAGS: -O3 -g -DUNITTEST -IContrib/catch2 -fsanitize=address
  Subset:   full build

But doing the same with CXXFLAGS specified from the caller yields a build environment where all CXXFLAGS are set to the caller's:

~/git/openMSX $ make CXXFLAGS="user flags" OPENMSX_FLAVOUR=unittest
Using Python: python3
A CXXFLAGS: user flags
B CXXFLAGS: user flags
C CXXFLAGS: user flags
D CXXFLAGS: user flags
Build configuration:
  Platform: x86_64-linux
  Flavour:  unittest
  Compiler: x86_64-pc-linux-gnu-g++
  CXXFLAGS: user flags
  Subset:   full build

This doesn't only affect the unittest build, but it does affect it somewhat acutely because it ends up overwriting the -DUNITTEST setting, which breaks some tests.