c-api.test fails on MS-Windows due to non-portable quoting

[Eli Zaretskii]

It fails like this:

     Running c-api.test
     'CUR' is not recognized as an internal or external command,
     operable program or batch file.
     egrep: Unmatched ( or \('CUR' is not recognized as an internal or external 
command, operable program or batch file.

This is because it quotes shell commands /bin/sh '..' style:

     (define (egrep string filename)
       (zero? (system (string-append "egrep '" string "' " filename
                                     " >" %null-device))))

The solution is to use the ".." style of quoting:

--- test-suite/tests/c-api.test~0       2016-01-02 13:32:40.000000000 +0200
+++ test-suite/tests/c-api.test 2016-07-23 14:12:57.257375000 +0300
@@ -22,7 +22,7 @@
 (define srcdir (cdr (assq 'srcdir %guile-build-info)))
 
 (define (egrep string filename)
-  (zero? (system (string-append "egrep '" string "' " filename
+  (zero? (system (string-append "egrep \"" string "\" " filename
                                 " >" %null-device))))
 
 (define (seek-offset-test dirname)

OK to push such a change?