Change default CONFIG to none

- Use default value of `CXX` instead of forcing override to `clang++`.
- Add base `CXXFLAGS` and `ABCMKARGS` in else condition of `ifeq ($(CONFIG),..)`
  block and output the value of `CXX`.
- Change readme to mention `CXX` envvar and that using `make config-clang` etc
  will ignore `CXX`.

Makefile

@@ -1,5 +1,6 @@
 
-CONFIG := clang
+CONFIG := none
+# CONFIG := clang
 # CONFIG := gcc
 # CONFIG := afl-gcc
 # CONFIG := emcc
@@ -362,6 +363,10 @@ EXE = .exe
 
 else ifneq ($(CONFIG),none)
 $(error Invalid CONFIG setting '$(CONFIG)'. Valid values: clang, gcc, emcc, mxe, msys2-32, msys2-64)
+else
+$(info Using provided C++ compiler, '$(CXX)'.)
+CXXFLAGS += -std=$(CXXSTD) -Os
+ABCMKARGS += ARCHFLAGS="-DABC_USE_STDINT_H $(ABC_ARCHFLAGS)"
 endif
 
 ifeq ($(ENABLE_LIBYOSYS),1)

README.md

@@ -105,11 +105,17 @@ For Cygwin use the following command to install all prerequisites, or select the
 
 	setup-x86_64.exe -q --packages=bison,flex,gcc-core,gcc-g++,git,libffi-devel,libreadline-devel,make,pkg-config,python3,tcl-devel,boost-build,zlib-devel
 
-To configure the build system to use a specific compiler, use one of
+The environment variable `CXX` can be used to control the C++ compiler used, or
+run one of the following:
 
 	$ make config-clang
 	$ make config-gcc
 
+Note that these will result in `make` ignoring the `CXX` environment variable,
+unless `CXX` is assigned in the call to make, e.g.
+
+  $ make CXX=$CXX
+
 For other compilers and build configurations it might be
 necessary to make some changes to the config section of the
 Makefile.