package/pistache: new package

package/pistache: new package

The upstream project http://pistache.io/ does not provide release
tarballs. However, commit 73f248acd6db4c53 has been successfully
used for some time now.

The use of BR2_TOOLCHAIN_GCC_AT_LEAST_4_9 is for C++14 support.
Tested it with gcc-linaro-4.9-2016.02-x86_64_arm-linux-gnueabi
and buildroot's ./utils/test-pkg

Pistache does not build with uClibc, the macro RUSAGE_THREAD for
getrusage() is not properly declared. This is an upstream issue:
https://github.com/oktal/pistache/issues/175
Hence the dependecy !BR2_TOOLCHAIN_USES_UCLIBC

Signed-off-by: Thomas Ruschival <thomas@ruschival.de>
[yann.morin.1998@free.fr:
  - fix PISTACHE_LICENSE_FILES (missing trailing 'S')
  - add hash for license file
  - two spaces in license file
  - tweaks in commit log
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>