Protect digamma tests behind a EIGEN_HAS_C99_MATH check.
Protect digamma tests behind a EIGEN_HAS_C99_MATH check.

ritsu1228/eigen,ritsu1228/eigen,ROCmSoftwarePlatform/hipeigen,pasuka/eigen,pasuka/eigen,pasuka/eigen,ROCmSoftwarePlatform/hipeigen,ritsu1228/eigen,pasuka/eigen,ritsu1228/eigen,ritsu1228/eigen,ROCmSoftwarePlatform/hipeigen,ROCmSoftwarePlatform/hipeigen,pasuka/eigen