How do I stop autotools from adding command line options? - c++

Question

In my source directory, I type make main.o, which issues the command:
g++ -DHAVE_CONFIG_H -I. -I.. -std=gnu++11 -fpermissive -Wno-write-strings -O0 -g -g -O2 -MT main.o -MD -MP -MF .deps/main.Tpo -c -o main.o main.cc
mv -f .deps/main.Tpo .deps/main.Po
Some of this was created by me (-std=gnu++11 -fpermissive -Wno-write-strings -O0 -g) and some of it wasn't (-g -O2 -MT main.o -MD -MP -MF .deps/main.Tpo)
Q1: How do I tell autotools to stop inserting -g -O2?
Q2: What's the deal with -MT main.o -MD -MP -MF .deps/main.Tpo? Is it necessary? Unnecessary? Can I get rid of it? If so, how?
Q3: How do I add to AM_CXXFLAGS via m4 macros seemlessly? The approach I have adopted so far is to define the macro m4/ax_debug.m4 as follows:
AC_DEFUN([AX_DEBUG], [
AC_ARG_WITH([debug],
[AS_HELP_STRING([--with-debug], [Enable debug (default is no)])],
[with_debug=$withval],
[with_debug=no])
AM_CONDITIONAL([WITH_DEBUG], [test "x$with_debug" = "xyes"])
AS_IF([test "x$with_debug" = "xyes"], [
AC_SUBST([DEBUG_CXXFLAGS], ["-O0 -g"])
])
AC_MSG_NOTICE([checking if debugging required ... $with_debug])
])dnl AX_DEBUG
and to use those flags within src/Makefile.am as follows:
AM_CXXFLAGS = ... $(DEBUG_CXXFLAGS)
Is there a way I can arrange it so that DEBUG_CXXFLAGSis added automatically?
Update (with possible answers): In regards to Q1, the answer appears to be to add the following line to configure.ac after AC_INIT(...):
: ${CXXFLAGS=""}
and delete $(DEBUG_CXXFLAGS) from src/Makefile.am.
In regards to Q2, I still have no idea what that's about.
In regards to Q3, the answer would appear to be to write
AC_SUBST([CXXFLAGS], ["$CXXFLAGS -O0 -g"])
instead of
AC_SUBST([DEBUG_CXXFLAGS], ["-O0 -g"])
I'm not sure if that's an "official" solution though.