Autotools: optimize with -Og when debugging

When debug (-g) is enabled (default when compiling a development
version), the existing -O optimization level is not correct, since
many variables are optimized out when debugging.

Use -Og instead, and condition not on compiling a development version,
but on --enable-debug. This is the same by default, but it more
precise in our case.

INSTALL

@@ -107,7 +107,7 @@ flags:
     the settings in terms of various options that are described later
 
                       release   prerelease  development profiling gprof
-    optimization        -O2         -O2         -O         -O2     -O2
+    optimization        -O2         -O2         -Og        -O2     -O2
     assertions                       X           X
     stdlib-assertions                            X
     stdlib-debug
@@ -213,10 +213,11 @@ values):
 
   o --enable-optimization=VALUE enables you to set optimization to a
     higher level than the default, for example --enable-optimization=-O3.
+    Default is -Og when debugging is enabled, -O2 otherwise.
 
   o --disable-optimization - you can use this to disable compiler
     optimization of LyX. The compile may be much quicker with some
-    compilers, but LyX will run more slowly.
+    compilers, but LyX will be slower.
 
   o --enable-debug will add debug information to your binary. This
     requires a lot more disk space, but is a must if you want to try

config/lyxinclude.m4

@@ -326,8 +326,8 @@ AC_ARG_ENABLE(optimization,
     enable_optimization=yes;)
 case $enable_optimization in
     yes)
-        if test $lyx_devel_version = yes ; then
-            lyx_optim=-O
+        if test $enable_debug = yes ; then
+            lyx_optim=-Og
         else
             lyx_optim=-O2
         fi;;