CDRIVER-3906 do not add libm on macOS

[kevinAlbs]

Requiring consumers of libbson-static to link against libm on macOS causes an issue for the Go driver's use of libmongocrypt (error encountered originally in MONGOCRYPT-309, but copying details here).

$ pkg-config --libs --cflags libmongocrypt-static
-DMONGOCRYPT_STATIC_DEFINE -DKMS_MSG_STATIC -DBSON_STATIC -I/usr/local/Cellar/libmongocrypt/1.2.0/include/mongocrypt -I/usr/local/Cellar/mongo-c-driver/1.17.4/include/libbson-1.0 -L/usr/local/Cellar/mongo-c-driver/1.17.4/lib /usr/local/Cellar/libmongocrypt/1.2.0/lib/libmongocrypt-static.a /usr/local/Cellar/libmongocrypt/1.2.0/lib/libkms_message-static.a -framework CoreFoundation -framework Security -lbson-static-1.0 /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/lib/libm.tbd

The /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/lib/libm.tbd is a valid input to the macOS linker, and is a text based stub-library. By default, Go rejects unrecognized linker inputs. This will be fixed in golang/go#44263 in the go command.

Example:

// bson.go
package bson

// #cgo pkg-config: libbson-static-1.0
// #include <bson/bson.h>
// #include <stdlib.h>
import "C"
import "fmt"

func RunBson() {
	v := C.bson_get_version()
	fmt.Printf("v=%v\n", v)
}

Attempting to build with go results in an error:

$ go build -o bson bson.go
go build command-line-arguments: invalid flag in pkg-config --libs: /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/lib/libm.tbd

Users can work around this by specifying the environment variable CGO_LDFLAGS_ALLOW='^.*tbd$'. But, libm is part of libsystem on macOS. So I do not think it is necessary to explicitly link to libm on macOS.

[rcsanchez97]

LGTM