valgrind: fix uclibc c99 support detection

Fixes [1]:

  checking for /home/peko/autobuild/instance-0/output/host/bin/powerpc-linux-gcc option to accept ISO C99... unsupported
  configure: error: Valgrind relies on a C compiler supporting C99

with the following in the valgrind-3.14.0/config.log:

  configure:5517: checking for .../host/bin/powerpc-linux-gcc option to accept ISO C99
  configure:5666: .../host/bin/powerpc-linux-gcc -c -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -fno-stack-protector -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 conftest.c >&5
  conftest.c:55:9: error: unknown type name 'wchar_t'
     const wchar_t *name;
           ^~~~~~~

[1] http://autobuild.buildroot.net/results/b25013f785a11f07e8da3735741b96036712f42a

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

package/valgrind/valgrind.mk

@@ -36,6 +36,9 @@ endif
 
 VALGRIND_CONF_ENV = CFLAGS="$(VALGRIND_CFLAGS)"
 
+# fix uclibc configure c99 support detection
+VALGRIND_CONF_ENV += ac_cv_prog_cc_c99='-std=gnu99'
+
 # On ARM, Valgrind only supports ARMv7, and uses the arch part of the
 # host tuple to determine whether it's being built for ARMv7 or
 # not. Therefore, we adjust the host tuple to specify we're on