Add cmake option to specify whether warnings / errors are enabled by default.

Currently warnings / errors are enabled by default in debug build.
Otherwise they could be enabled only via environmental variable or
API option call. Add possibility to specify the default during the
build time. This simplifies e.g. integration of the library into
bigger projects as no source changes would be required.

src/options.c

@@ -51,7 +51,7 @@ typedef struct mi_option_desc_s {
 static mi_option_desc_t options[_mi_option_last] =
 {
   // stable options
-  { MI_DEBUG, UNINIT, MI_OPTION(show_errors) },
+  { MI_DEBUG | MI_SHOW_ERRORS, UNINIT, MI_OPTION(show_errors) },
   { 0, UNINIT, MI_OPTION(show_stats) },
   { 0, UNINIT, MI_OPTION(verbose) },