Add NOOPT make option to disable optimization

GCC recommends use of -Og for debug builds, which theoretically
disables optimizations that will cause problems for debugging, but
leaves some on to improve performance.  Presently, in non-release
builds, the Makefile always uses -Og if supported by the compiler.
Unfortunately, it appears that even -Og permits control flow to --
from the standpoint of someone using a debugger -- not step a single
line at a time when stepping, as lines may have been optimized out.

Provide a make option NOOPT, so that if someone really needs -O0
builds, they can get them from the command line without patching the
Makefile each time (and possibly letting that change slip into pull
requests).

Makefile

@@ -63,6 +63,8 @@
 #  make DYNAMIC_LINKING=1
 # Use MSYS2 as the build environment on Windows
 #  make MSYS2=1
+# Turn off all optimizations, even debug-friendly optimizations
+#  make NOOPT=1
 # Astyle all source files.
 #  make astyle
 # Check if source files are styled properly.
@@ -303,10 +305,18 @@ ifdef RELEASE
 endif
 
 ifndef RELEASE
-  ifeq ($(shell $(CXX) -E -Og - < /dev/null > /dev/null 2>&1 && echo fog),fog)
-    OPTLEVEL = -Og
-  else
+  ifdef NOOPT
+    # While gcc claims to include all information required for
+    # debugging at -Og, at least with gcc 8.3, control flow
+    # doesn't move line-by-line at -Og.  Provide a command-line
+    # way to turn off optimization (make NOOPT=1) entirely.
     OPTLEVEL = -O0
+  else
+    ifeq ($(shell $(CXX) -E -Og - < /dev/null > /dev/null 2>&1 && echo fog),fog)
+      OPTLEVEL = -Og
+    else
+      OPTLEVEL = -O0
+    endif
   endif
   CXXFLAGS += $(OPTLEVEL)
 endif