COMP: External_curl - Explicitly pass -fPIC flag on x86_64 platform

This commit will fix the following link error:

/usr/bin/ld: /home/jchris/Projects/Slicer-SuperBuild-Debug-Qt485-new-buildsystem/curl-build/lib/libcurl.a(easy.c.o): relocation R_X86_64_32S against `malloc' can not be used when making a shared object; recompile with -fPIC
/home/jchris/Projects/Slicer-SuperBuild-Debug-Qt485-new-buildsystem/curl-build/lib/libcurl.a: error adding symbols: Bad value
collect2: error: ld returned 1 exit status

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22733 3bd1e089-480b-0410-8dfb-8563597acbee