Add fPIC flag on x86_64 if the compiler supports it.

Fixes #34.

double-conversion/CMakeLists.txt

-
 set(headers
   bignum.h
   cached-powers.h
@@ -25,6 +24,14 @@ ${headers}
 
 target_include_directories(double-conversion PUBLIC $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/..>)
 
+# Add fPIC on x86_64 when supported.
+include(CheckCXXCompilerFlag)
+check_cxx_compiler_flag(-fPIC CXX_HAS_FPIC)
+
+if(CXX_HAS_FPIC AND CMAKE_SYSTEM_PROCESSOER STREQUAL "x86_64")
+  set_target_properties(double-conversion PROPERTIES COMPILE_FLAGS "-fPIC")
+endif()
+
 #
 # associates the list of headers with the library
 # for the purposes of installation/import into other projects