ncbi-blast+: Unbreak build with gcc8 following gcc9 patch

GCC 8 and 9 are incompatible in handling const variables with OpenMP
default(none). All const variables default to shared even with default(none)
present. 8 errors out if a const variable is explicitly defined as shared and
9 requires an explicit definition.

Using an upstream trunk patch for now, which checks the compiler version.
Also discussing more portable approaches to avoid future issues.

Reported by: jrm