help description added for recently introduced gmake LEXFLAGS option

Change-Id: Ie94ba4b3a774ab947574c74837ecbc932bb3b621

solenv/gbuild/gbuild.help.txt

@@ -67,6 +67,7 @@ INTERACTIVE VARIABLES:
                        If not empty enable the RTL_LOGFILE_* time logging facility.
                        export RTL_LOGFILE=rtl_logfile.nopid when running office, to
                        get timestamps written out.
+       LEXFLAGS        Add as flags for LEX scanner generator invocation.
        YACCFLAGS       Add as flags for YACC parser generator invocation.
        CPPFLAGS        Add as preprocessor flags for C/C++/ObjC/ObjC++ compilation.
        CFLAGS          Add as compiler flags for plain C compilation.