Fix building when lex source refers to a file generated by yacc

Scan lex and yacc sources for dependencies as if they were C source code,
which is close enough to reality.

This will unfortunately result in the generated source files depending on
the generated headers, while it should have been the object files created
from these sources which have that dependency. But qmake cannot do better,
and this is good enough.

Task-number: QTBUG-56507
Change-Id: Ic3e1941bf2e2820bfddf99deba854e1e82f83669
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>

mkspecs/features/lex.prf

@@ -5,6 +5,7 @@
 {
     lex.name = Lex ${QMAKE_FILE_IN}
     lex.input = LEXSOURCES
+    lex.dependency_type = TYPE_C
     lex_included {
         lex.CONFIG += no_link
     } else {

mkspecs/features/yacc.prf

@@ -35,6 +35,7 @@
     yacc_impl.name = source for ${QMAKE_FILE_IN}
     yacc_impl.input = YACCSOURCES
     yacc_impl.variable_out = GENERATED_SOURCES
+    yacc_impl.dependency_type = TYPE_C
     yacc_impl.commands = $$escape_expand(\\n)        # We don't want any commands where, but if command is empty no rules are created
     yacc_impl.depends = $${QMAKE_CPP_MOD_MOC}${QMAKE_FILE_BASE}$${QMAKE_MOD_YACC}$${first(QMAKE_EXT_H)}    # Make sure we depend on the step above
     yacc_impl.output = $${QMAKE_CPP_MOD_MOC}${QMAKE_FILE_BASE}$${QMAKE_MOD_YACC}$${first(QMAKE_EXT_CPP)}   # Faked output from this step, output really created in step above