diagnostics: Fix compile error for MinGW <7.0

diagnostics: Fix compile error for MinGW <7.0

The define ENABLE_VIRTUAL_TERMINAL_PROCESSING was introduced in MinGW
7.0

Build failure when building with MinGW 5.0.3:

.../gcc/diagnostic-color.cc:
In function 'bool should_colorize()':
.../gcc/diagnostic-color.cc:317:41:
error: 'ENABLE_VIRTUAL_TERMINAL_PROCESSING' was not declared in this
scope
       mode |= ENABLE_PROCESSED_OUTPUT | ENABLE_VIRTUAL_TERMINAL_PROCESSING;
                                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.../gcc/diagnostic-color.cc:317:41:
note: suggested alternative: 'ENABLE_RTL_FLAG_CHECKING'
       mode |= ENABLE_PROCESSED_OUTPUT | ENABLE_VIRTUAL_TERMINAL_PROCESSING;
                                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                         ENABLE_RTL_FLAG_CHECKING
.../gcc/diagnostic-color.cc:
In function 'bool auto_enable_urls()':
.../gcc/diagnostic-color.cc:407:50:
error: 'ENABLE_VIRTUAL_TERMINAL_PROCESSING' was not declared in this
scope
   if (GetConsoleMode (handle, &mode) && !(mode & ENABLE_VIRTUAL_TERMINAL_PROCESSING))
                                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.../gcc/diagnostic-color.cc:407:50:
note: suggested alternative: 'ENABLE_RTL_FLAG_CHECKING'
   if (GetConsoleMode (handle, &mode) && !(mode & ENABLE_VIRTUAL_TERMINAL_PROCESSING))
                                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                                  ENABLE_RTL_FLAG_CHECKING
Makefile:1195: recipe for target 'diagnostic-color.o' failed
make[1]: *** [diagnostic-color.o] Error 1

gcc/ChangeLog:

	* diagnostic-color.cc: Conditionally enable terminal processing
	based on define availability.
	* pretty-print.cc: Likewise.

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>