CDRIVER-4776 add thread library to pkg-config Libs

[kevinAlbs]

Summary

Add thread library to pkg-config Libs

Background & Motivation

Attempting to compile and link in separate steps with static libbson results in a linker error related to pthread with gcc 7.5.0:

# Tell pkg-config where to find .pc files:
export PKG_CONFIG_PATH=$(pwd)/.install-baseline/lib/pkgconfig
# Compile:
gcc -c ./src/libbson/examples/hello_bson.c -o hello_bson.o $(pkg-config --cflags libbson-static-1.0)
# Link:
gcc -o hello_bson hello_bson.o $(pkg-config --libs libbson-static-1.0)

Results in an error at the Link step:

/usr/bin/ld: /home/ubuntu/code/tasks/mongo-c-driver-C4776/.install/lib/libbson-static-1.0.a(common-b64.c.o): undefined reference to symbol 'pthread_once@@GLIBC_2.2.5'
//lib/x86_64-linux-gnu/libpthread.so.0: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status

man gcc notes for -pthread:

should be used consistently for both compilation and linking.

After applying these changes, the Libs section of the installed libbson-static-1.0.pc includes -pthread. The reproducing steps pass.