Restore the missing -P option for lex handling

We don't know why it works, but we've been told that it fixes
some builds and we have no reason to think it will break others.

Task-number: QTBUG-52998
Change-Id: Iacaa1fb3c49e4d7aafd4167e591ff3724489872e
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

mkspecs/features/lex.prf

@@ -11,9 +11,13 @@
         lex.variable_out = GENERATED_SOURCES
     }
 
+    isEmpty(QMAKE_LEXFLAGS_MANGLE):QMAKE_LEXFLAGS_MANGLE = -P${QMAKE_FILE_BASE}
+    QMAKE_LEXEXTRAFLAGS = $$QMAKE_LEXFLAGS
+    !yacc_no_name_mangle:QMAKE_LEXEXTRAFLAGS += $$QMAKE_LEXFLAGS_MANGLE
+
     contains(QMAKE_LEX, .*flex) {
         # GNU flex, we can use -o outfile
-        lex.commands = $$QMAKE_LEX $$QMAKE_LEXFLAGS --nounistd -o ${QMAKE_FILE_OUT} ${QMAKE_FILE_IN}
+        lex.commands = $$QMAKE_LEX $$QMAKE_LEXEXTRAFLAGS --nounistd -o ${QMAKE_FILE_OUT} ${QMAKE_FILE_IN}
     } else {
         # stupid POSIX lex, it only generates a file called lex.yy.c
         # or lex.prefix.c if the -P<prefix> option is active