sblas3: Exclude from double precision build

[jschueller]

When only BUILD_DOUBLE is enabled (without BUILD_SINGLE), the single-precision Level 3 BLAS routines (sgemm, ssymm, ssyrk, strsm, etc.) are still compiled into the BLAS library. This appears to be a copy-paste error — SBLAS3 should not be in the BUILD_DOUBLE section. While the object files may be ignored at link time, they bloat the build and may cause unexpected symbol exports.

[ACSimon33]

I think we need the single precision BLAS routines because if we build LAPACK with only double precision, these single precision routines are also compiled (SRC/CMakeLists.txt:166):

set(DSLASRC
    sgetrf.f sgetrf2.f sgetrs.f sisnan.f slaisnan.f slaswp.f spotrf.f spotrf2.f
    spotrs.f)

Those are needed for the mixed precision routines, like dsposv.

We could build those only if both single and double precision are enabled. Or we introduce a new CMake variable BUILD_MIXED_PRECISION ...