build: Use @FGREP@ by configure for libgcrypt-config.

* configure.ac (AC_PROG_FGREP): Add.
* src/libgcrypt-config.in: Use @FGREP@.

We now have AC_PROG_FGREP in configure.ac, which has been indirectly
used by libtool.

• GnuPG-bug-id: T6964
• Reported-by: Dirk Mueller
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>