libv4l: fix uclibc-ng configure/compile

libv4l: fix uclibc-ng configure/compile

With raspberrypi_defconfig:

  libv4l2rds.c:256:2: error: 'for' loop initial declarations are only allowed in C99 or C11 mode
    for (int i = 0; i < tuning->station_cnt; i++) {
    ^
  libv4l2rds.c:256:2: note: use option -std=c99, -std=gnu99, -std=c11 or -std=gnu11 to compile your code

>From build/libv4l-1.6.2/config.log:

configure:4709: checking for .../host/usr/bin/arm-buildroot-linux-uclibcgnueabi-gcc option to accept ISO C99
configure:4858: .../host/usr/bin/arm-buildroot-linux-uclibcgnueabi-gcc -std=gnu99 -c -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64   -Os  -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 conftest.c >&5
conftest.c:54:9: error: unknown type name 'wchar_t'
   const wchar_t *name;

The C99 detection problem seems more related to missing wchar_t type support than the compiler option?

Adding LIBV4L_CONF_ENV = ac_cv_prog_cc_c99='-std=c99' gives a lot of compile errors like:

libv4lconvert.c: In function 'dev_ioctl':
processing/../libv4lsyscall-priv.h:85:10: error: 'SYS_ioctl' undeclared (first use in this function)
  syscall(SYS_ioctl, (int)(fd), (unsigned long)(cmd), (void *)(arg))
          ^
libv4lconvert.c:43:9: note: in expansion of macro 'SYS_IOCTL'
  return SYS_IOCTL(fd, cmd, arg);
         ^
processing/../libv4lsyscall-priv.h:85:10: note: each undeclared identifier is reported only once for each function it appears in
  syscall(SYS_ioctl, (int)(fd), (unsigned long)(cmd), (void *)(arg))
          ^
libv4lconvert.c:43:9: note: in expansion of macro 'SYS_IOCTL'
  return SYS_IOCTL(fd, cmd, arg);
         ^

Adding LIBV4L_CONF_ENV = ac_cv_prog_cc_c99='-std=gnu99' solves all configure/compile errors.

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>