package/snappy: remove pthread handling for static builds

package/snappy: remove pthread handling for static builds

Commit 0e2ee0e6e8076 (package.snappy: fix static builds with threads)
added LIBS=-pthread to configure options to fix static build with
threads on some architectures. This works for autotools packages. Then,
commit d26282b3aa14cb (package/snappy: bump version, switch to cmake)
switched to cmake that does not understand the 'LIBS' parameter.

As pointed out by Thomas[1] the special handling of libpthread for
static builds is not needed anymore: it was needed only to build the
tests, and these are no longer built. Note that packages that link
statically with snappy will still need to link with -pthread. Only
two in-tree packages currently depend on snappy: lua-csnappy depends
on !BR2_STATIC_LIBS and leveldb adds -pthread from its build system.

Tested with the following musl & uclibc defconfigs which also fix build
errors:
http://autobuild.buildroot.net/results/39f/39f9fc4300ddca19e8a533765b46838003752000/
http://autobuild.buildroot.net/results/886/8860b9259f7a3aecb2ddbfa787be048072bd347b/

[1] http://lists.busybox.net/pipermail/buildroot/2017-July/199549.html

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[Baruch: improved commit message]
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
[Arnout: improved commit message more, based on Yann's input]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>