Add 'POSITION_INDEPENDENT_CODE ON' (also known as '-fPIE'). (#99)

First thx for these very useful lib!

I encountered some issues when linking with `libcpptrace`. Typically,
here is the error I got:
```
/usr/bin/ld: lib/cpptrace/lib/libcpptrace.a(cpptrace.cpp.o): relocation R_X86_64_32S against symbol `_ZTVNSt7__cxx1115basic_stringbufIcSt11char_traitsIcESaIcEEE@@GLIBCXX_3.4.21' can not be used when making a PIE object; recompile with -fPIE
```
I just added the `POSITION_INDEPENDENT_CODE ON` flag on the targets, and
now it works like a charm :-).

CMakeLists.txt

@@ -267,6 +267,7 @@ set_target_properties(
     SOVERSION "${PROJECT_VERSION_MAJOR}"
     EXPORT_NAME "cpptrace"
     OUTPUT_NAME "cpptrace"
+    POSITION_INDEPENDENT_CODE ${CPPTRACE_POSITION_INDEPENDENT_CODE}
 )
 
 # Header files generated by CMake

README.md

@@ -898,6 +898,7 @@ Other useful configurations:
   the fault of your project. Defaults to On.
 - `CPPTRACE_INSTALL_CMAKEDIR`: Override for the installation path for the cmake configs.
 - `CPPTRACE_USE_EXTERNAL_LIBDWARF=On/Off`: Get libdwarf from `find_package` rather than `FetchContent`.
+- `CPPTRACE_POSITION_INDEPENDENT_CODE=On/Off`: Compile the library as a position independent code (PIE). Defaults to On.
 
 Testing:
 - `CPPTRACE_BUILD_TESTING` Build small demo and test program

cmake/OptionVariables.cmake

@@ -164,6 +164,7 @@ option(CPPTRACE_CONAN "" OFF)
 option(CPPTRACE_VCPKG "" OFF)
 option(CPPTRACE_SANITIZER_BUILD "" OFF)
 option(CPPTRACE_WERROR_BUILD "" OFF)
+option(CPPTRACE_POSITION_INDEPENDENT_CODE "" ON)
 
 mark_as_advanced(
   CPPTRACE_BACKTRACE_PATH