Adding explicit link to libm for pure-C curve fitting example.

- Any pure-C program #including <math.h> will need to link against
  libm, some compilers will let an indirect link slide (via Ceres in
  this case) but some won't.

Change-Id: I6890702fa0d2c3fbb747f0f81fc3fa3631839de4

examples/CMakeLists.txt

diff --git a/examples/CMakeLists.txt b/examples/CMakeLists.txt
index 5205ccb..28eec9b 100644
--- a/examples/CMakeLists.txt
+++ b/examples/CMakeLists.txt
@@ -41,7 +41,10 @@
 TARGET_LINK_LIBRARIES(curve_fitting ceres)
 
 ADD_EXECUTABLE(curve_fitting_c curve_fitting.c)
-TARGET_LINK_LIBRARIES(curve_fitting_c ceres)
+# As this is a C file #including <math.h> we have to explicitly add the math
+# library (libm). Although some compilers (dependent upon options) will accept
+# the indirect link to libm via Ceres, at least GCC 4.8 on pure Debian won't.
+TARGET_LINK_LIBRARIES(curve_fitting_c ceres m)
 
 
 ADD_EXECUTABLE(robust_curve_fitting robust_curve_fitting.cc)