giflib: relocatable shared lib on macOS

[SpaceIm]

see conan-io/hooks#376

---

• I've read the guidelines for contributing.
• I've followed the PEP8 style guides for Python code in the recipes.
• I've used the latest Conan client version.
• I've tried at least one configuration locally with the
  conan-center hook activated.

[SpaceIm]

CMake Error at build/86b9715155a3ea9d377d41d1d937978b28e691c8/FindGIF.cmake:40 (add_library):
  Attempting to use @rpath without CMAKE_SHARED_LIBRARY_RUNTIME_C_FLAG being
  set.  This could be because you are using a Mac OS X version less than 10.5
  or because CMake's platform configuration is corrupt.
Call Stack (most recent call first):
  build/86b9715155a3ea9d377d41d1d937978b28e691c8/FindGIF.cmake:126 (conan_package_library_targets)
  CMakeLists.txt:7 (find_package)

It seems that it happens only with shared libs produced by autotools builds. => no, some CMake based recipes also have this issue.

[conan-center-bot]

All green in build 3 (359a467f98dcf9fd7326f1b3a1219370c8b5870d):

• giflib/5.1.4@:
  All packages built successfully! (All logs)

• giflib/5.2.1@:
  All packages built successfully! (All logs)