gbuild: Replace SHOWINCLUDES_PREFIX without regexp.

With MSVC Japanese, SHOWINCLUDES_PREFIX includes regexp special
charactor \x5b (left square bracket). In this case replacing
SHOWINCLUDES_PREFIX with regexp fails. To fix it, this commit
changes the script not to use regexp.

Example value of $SHOWINCLUDES_PREFIX from MSVC Japanese:
$ echo $SHOWINCLUDES_PREFIX | hexdump -C
83 81 83 82 3a 20 83 43 83 93 83 4e 83 8b 81 5b |....:.C...N...[|
83 68 20 83 74 83 40 83 43 83 8b 3a 20 20 43 3a |.h.t.@.C..:  C:|
5c 54 65 6d 70 5c 73 74 64 69 6f 2e 68 0d 0a    |\Temp\stdio.h..|

Change-Id: Ic7d85fc3a93807de11155488e9d0368427375c78
Reviewed-on: https://gerrit.libreoffice.org/3579
Reviewed-by: Kohei Yoshida <kohei.yoshida@suse.de>
Tested-by: Kohei Yoshida <kohei.yoshida@suse.de>

solenv/gbuild/platform/filter-showIncludes.awk

@@ -31,8 +31,6 @@ BEGIN {
     if (!showincludes_prefix) {
         showincludes_prefix = "Note: including file:"
     }
-    regex = "^ *" showincludes_prefix " *"
-    pattern = "/" regex "/"
 
     # to match especially drive letters in whitelist case insensitive
     IGNORECASE = 1
@@ -42,8 +40,10 @@ BEGIN {
 }
 
 {
-    if ($0 ~ regex) {
-        sub(regex, "")
+    sub(/^ */, "")
+    if (index($0, showincludes_prefix) == 1) {
+        $0 = substr($0, 1, length(showincludes_prefix))
+        sub(/^ */, "")
         gsub(/\\/, "/")
         gsub(/ /, "\\ ")
         if ($0 ~ whitelist) { # filter out system headers