pv: fix build failures when building under a path containing 'yes'

pv: fix build failures when building under a path containing 'yes'

Depending on the configuration, the cpp output may contain the string 'yes'
in a comment if built under a path containing 'yes', confusing the _AIX
test:

${CROSS}-cpp conftest.h
\# 1 "conftest.h"
\# 1 "<built-in>"
\# 1 "<command-line>"
\# 31 "<command-line>"
\# 1 "/home/peko/source/buildroot/output-yes/host/x86_64-buildroot-linux-gnu/sysroot/usr/include/stdc-predef.h"
\# 32 "<command-line>" 2
\# 1 "conftest.txt"

If misdetected, the configure script adds -lc128 to LIBS, causing the
AC_CHECKS_FUNCS check for stat64 to fail, which in turn causes compilation
errors about redefinition of symbols:

In file included from ./src/include/pv-internal.h:9:0,
                 from src/pv/file.c:5:
./src/include/config.h:76:18: error: redefinition of 'struct stat'
  #  define stat64 stat
                   ^
Fix it by only matching on 'yes' on a line by itself.

As pv doesn't cleanly autoreconf (it doesn't use automake and configure.in
is located in subdir), instead directly patch configure.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>