Use CMAKE_POSITION_INDEPENDENT_CODE=ON instead of -fPIE or -pie

[eramongodb]

Followup to #2097, which seems to have somehow broken the hardened-compile task:

/usr/lib/gcc/x86_64-redhat-linux/8/../../../../lib64/Scrt1.o: In function `_start':
(.text+0x24): undefined reference to `main'
collect2: error: ld returned 1 exit status
gmake[2]: *** [src/libbson/CMakeFiles/bson_shared.dir/build.make:550: src/libbson/libbson2.so.2.2.0] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:1549: src/libbson/CMakeFiles/bson_shared.dir/all] Error 2

My rough understanding of what is happening here is that the explicit -pie linker flag ("position-independent executable") being passed via LDFLAGS was being applied to shared library targets, which is incorrect (not an executable). It's not clear to me which part of #2097 is responsible for exposing this issue (newer CMake version? use of Ninja?). Nevertheless, this PR proposes avoiding the explicit compiler or linker flags by using the CMake CMAKE_POSITION_INDEPENDENT_CODE variable instead, allowing CMake to apply the appropriate flags according to each target.

The only remaining instance of -fPIE in the repo is:

mongo-c-driver/src/libmongoc/tests/test-mongoc-handshake.c

Line 390 in cfbcdb4

md->flags = bson_strdup("CFLAGS=\"-fPIE\"");

There are no more instances of the -pie flag. There are several instances of explicit -fPIC flags, but replacing these with POSITION_INDEPENDENT_CODE (target property) is deferred to another time (not essential to the goal of this PR).