Change -O3 to -g in CXXFLAGS_DEBUG for gfortran build target

The definition of CXXFLAGS_DEBUG in the gfortran build target previously
included the -O3 optimization flag that is more typical of non-debug flags,
and it lacked the -g flag. This commit simply replaces -O3 with -g in
the definition of CXXFLAGS_DEBUG for the gfortran build target.

Makefile

@@ -225,7 +225,7 @@ gfortran:
 	"LDFLAGS_OPT = -O3 -m64" \
 	"FFLAGS_DEBUG = -g -m64 -ffree-line-length-none -fconvert=big-endian -ffree-form -fbounds-check -fbacktrace -ffpe-trap=invalid,zero,overflow" \
 	"CFLAGS_DEBUG = -g -m64" \
-	"CXXFLAGS_DEBUG = -O3 -m64" \
+	"CXXFLAGS_DEBUG = -g -m64" \
 	"LDFLAGS_DEBUG = -g -m64" \
 	"FFLAGS_OMP = -fopenmp" \
 	"CFLAGS_OMP = -fopenmp" \