build: always set -g -O2 in CORE_CXXFLAGS

This avoids cases of missing -O2, when *FLAGS has been overriden.
Removes the need for duplicate code to clear autoconf defaults.

Also, move CORE_CXXFLAGS before DEBUG_CXXFLAGS, so that -O2 is always
overriden if debugging etc.

configure.ac

@@ -323,6 +323,15 @@ AC_ARG_ENABLE([external-signer],
 
 AC_LANG_PUSH([C++])
 
+dnl Always set -g -O2 in our CXXFLAGS. Autoconf will try and set CXXFLAGS to "-g -O2" by default,
+dnl so we suppress that (if CXXFLAGS hasn't been overridden by the user), given we are adding it
+dnl ourselves.
+CORE_CXXFLAGS="$CORE_CXXFLAGS -g -O2"
+
+if test "$CXXFLAGS_overridden" = "no"; then
+  CXXFLAGS=""
+fi
+
 dnl Check for a flag to turn compiler warnings into errors. This is helpful for checks which may
 dnl appear to succeed because by default they merely emit warnings when they fail.
 dnl
@@ -347,12 +356,6 @@ case $host in
 esac
 
 if test "$enable_debug" = "yes"; then
-  dnl If debugging is enabled, and the user hasn't overridden CXXFLAGS, clear
-  dnl them, to prevent autoconfs "-g -O2" being added. Otherwise we'd end up
-  dnl with "-O0 -g3 -g -O2".
-  if test "$CXXFLAGS_overridden" = "no"; then
-  CXXFLAGS=""
-  fi
 
   dnl Disable all optimizations
   AX_CHECK_COMPILE_FLAG([-O0], [DEBUG_CXXFLAGS="$DEBUG_CXXFLAGS -O0"], [], [$CXXFLAG_WERROR])
@@ -859,12 +862,6 @@ if test "$use_lcov" = "yes"; then
     [AC_MSG_ERROR([lcov testing requested but --coverage linker flag does not work])])
   AX_CHECK_COMPILE_FLAG([--coverage],[CORE_CXXFLAGS="$CORE_CXXFLAGS --coverage"],
     [AC_MSG_ERROR([lcov testing requested but --coverage flag does not work])])
-  dnl If coverage is enabled, and the user hasn't overridden CXXFLAGS, clear
-  dnl them, to prevent autoconfs "-g -O2" being added. Otherwise we'd end up
-  dnl with "--coverage -Og -O0 -g -O2".
-  if test "$CXXFLAGS_overridden" = "no"; then
-  CXXFLAGS=""
-  fi
   CORE_CXXFLAGS="$CORE_CXXFLAGS -Og -O0"
 fi
 
@@ -1996,7 +1993,7 @@ echo "  CC              = $CC"
 echo "  CFLAGS          = $PTHREAD_CFLAGS $CFLAGS"
 echo "  CPPFLAGS        = $DEBUG_CPPFLAGS $HARDENED_CPPFLAGS $CORE_CPPFLAGS $CPPFLAGS"
 echo "  CXX             = $CXX"
-echo "  CXXFLAGS        = $DEBUG_CXXFLAGS $HARDENED_CXXFLAGS $WARN_CXXFLAGS $NOWARN_CXXFLAGS $ERROR_CXXFLAGS $GPROF_CXXFLAGS $CORE_CXXFLAGS $CXXFLAGS"
+echo "  CXXFLAGS        = $CORE_CXXFLAGS $DEBUG_CXXFLAGS $HARDENED_CXXFLAGS $WARN_CXXFLAGS $NOWARN_CXXFLAGS $ERROR_CXXFLAGS $GPROF_CXXFLAGS $CXXFLAGS"
 echo "  LDFLAGS         = $PTHREAD_LIBS $HARDENED_LDFLAGS $GPROF_LDFLAGS $CORE_LDFLAGS $LDFLAGS"
 echo "  AR              = $AR"
 echo "  ARFLAGS         = $ARFLAGS"

src/Makefile.am

@@ -9,7 +9,7 @@ print-%: FORCE
 DIST_SUBDIRS = secp256k1
 
 AM_LDFLAGS = $(LIBTOOL_LDFLAGS) $(HARDENED_LDFLAGS) $(GPROF_LDFLAGS) $(SANITIZER_LDFLAGS) $(CORE_LDFLAGS)
-AM_CXXFLAGS = $(DEBUG_CXXFLAGS) $(HARDENED_CXXFLAGS) $(WARN_CXXFLAGS) $(NOWARN_CXXFLAGS) $(ERROR_CXXFLAGS) $(GPROF_CXXFLAGS) $(SANITIZER_CXXFLAGS) $(CORE_CXXFLAGS)
+AM_CXXFLAGS = $(CORE_CXXFLAGS) $(DEBUG_CXXFLAGS) $(HARDENED_CXXFLAGS) $(WARN_CXXFLAGS) $(NOWARN_CXXFLAGS) $(ERROR_CXXFLAGS) $(GPROF_CXXFLAGS) $(SANITIZER_CXXFLAGS)
 AM_CPPFLAGS = $(DEBUG_CPPFLAGS) $(HARDENED_CPPFLAGS) $(CORE_CPPFLAGS)
 AM_LIBTOOLFLAGS = --preserve-dup-deps
 PTHREAD_FLAGS = $(PTHREAD_CFLAGS) $(PTHREAD_LIBS)