Restore configure command line compatibility for PCRE support

Commit 9ca635482d renamed the configure
feature from pcre to pcre2, which unfortunately means that people would
have to change their build scripts to pass -qt-pcre2 instead of
-qt-pcre. As the configure check already verifies the correct PCRE
version for use (when using the system library), it seems more
convenient for our users to retain compatibility and call the configure
feature "pcre" again.

Change-Id: If26e7888814d8204d43baf7298d9916a4f856a48
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>

config_help.txt

@@ -206,7 +206,7 @@ Core options:
   -inotify ............. Enable inotify support
   -iconv ............... Enable iconv(3) support [posix/sun/gnu/no] (Unix only)
   -icu ................. Enable ICU support [auto]
-  -pcre2 ............... Select used libpcre2 [system/qt]
+  -pcre ................ Select used libpcre2 [system/qt]
   -pps ................. Enable PPS support [auto] (QNX only)
   -zlib ................ Select used zlib [system/qt]
                          ZLIB_LIBS=

src/corelib/configure.json

@@ -11,7 +11,7 @@
             "icu": "boolean",
             "inotify": "boolean",
             "journald": "boolean",
-            "pcre2": { "type": "enum", "values": [ "qt", "system" ] },
+            "pcre": { "type": "enum", "values": [ "qt", "system" ] },
             "posix-ipc": { "type": "boolean", "name": "ipc_posix" },
             "pps": { "type": "boolean", "name": "qqnx_pps" },
             "slog2": "boolean",
@@ -309,8 +309,8 @@
         },
         "system-pcre2": {
             "label": "Using system PCRE2",
-            "disable": "input.pcre2 == 'qt'",
-            "enable": "input.pcre2 == 'system'",
+            "disable": "input.pcre == 'qt'",
+            "enable": "input.pcre == 'system'",
             "condition": "libs.pcre2",
             "output": [
                 "privateFeature",