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ARROW-4500: [C++] Remove pthread / librt hacks causing linking issues in some Linux environments

pthread and librt are already part of the public link interface to `arrow_static` -- tagging on these additional link options can cause the libraries to get passed multiple times to the linker, or in the wrong order, causing link failures in certain environments

```
FAILED: debug/plasma_store_server
: && /usr/bin/ccache
/home/wesm/miniconda/envs/arrow-3.7/bin/x86_64-conda_cos6-linux-gnu-c++
-Wno-noexcept-type -fvisibility-inlines-hidden -std=c++17
-fmessage-length=0 -march=nocona -mtune=haswell -ftree-vectorize
-fPIC -fstack-protector-strong -fno-plt -O2 -pipe -ggdb -O0 -Wall
-Wconversion -Wno-sign-conversion -Werror -msse4.2
-fno-omit-frame-pointer -fPIC -g -Wl,-O2 -Wl,--sort-common
-Wl,--as-needed -Wl,-z,relro -Wl,-z,now -Wl,--disable-new-dtags
-rdynamic
src/plasma/CMakeFiles/plasma_store_server.dir/external_store.cc.o
src/plasma/CMakeFiles/plasma_store_server.dir/hash_table_store.cc.o
src/plasma/CMakeFiles/plasma_store_server.dir/store.cc.o -o
debug/plasma_store_server
-Wl,-rpath,/home/wesm/cpp-toolchain/lib: -lrt debug/libplasma.a
debug/libarrow.a -lrt
/home/wesm/cpp-toolchain/lib/libdouble-conversion.a
/home/wesm/cpp-toolchain/lib/libbrotlidec-static.a
/home/wesm/cpp-toolchain/lib/libbrotlienc-static.a
/home/wesm/cpp-toolchain/lib/libbrotlicommon-static.a
/home/wesm/cpp-toolchain/lib/libbz2.a
/home/wesm/cpp-toolchain/lib/liblz4.a
/home/wesm/cpp-toolchain/lib/libsnappy.a
/home/wesm/cpp-toolchain/lib/libz.so
/home/wesm/cpp-toolchain/lib/libzstd.a
/home/wesm/cpp-toolchain/lib/libglog.a
/home/wesm/cpp-toolchain/lib/libgflags.a
/home/wesm/cpp-toolchain/lib/libboost_system.so
/home/wesm/cpp-toolchain/lib/libboost_filesystem.so
/home/wesm/cpp-toolchain/lib/libboost_regex.so
jemalloc_ep-prefix/src/jemalloc_ep/dist//lib/libjemalloc_pic.a
/home/wesm/cpp-toolchain/lib64/libflatbuffers.a -pthread && :
/home/wesm/miniconda/envs/arrow-3.7/bin/../lib/gcc/x86_64-conda_cos6-linux-gnu/7.3.0/../../../../x86_64-conda_cos6-linux-gnu/bin/ld:
jemalloc_ep-prefix/src/jemalloc_ep/dist//lib/libjemalloc_pic.a(nstime.pic.o):
undefined reference to symbol 'clock_gettime@@GLIBC_2.2.5'
/home/wesm/miniconda/envs/arrow-3.7/bin/../lib/gcc/x86_64-conda_cos6-linux-gnu/7.3.0/../../../../x86_64-conda_cos6-linux-gnu/bin/ld:
/home/wesm/miniconda/envs/arrow-3.7/bin/../x86_64-conda_cos6-linux-gnu/sysroot/usr/lib/../lib/librt.so:
error adding symbols: DSO missing from command line

```

Author: Wes McKinney <wesm+git@apache.org>

Closes #3583 from wesm/ARROW-4500 and squashes the following commits:

db53dc9ee <Wes McKinney> Support ARROW_BUILD_STATIC=OFF
4a189ea87 <Wes McKinney> Simplify plasma library link libraries
b1d2a27ca <Wes McKinney> Remove pthread / librt hacks causing linking issues on Ubuntu 18.10