configure.ac: Compute the required llvm static libraries only once

configure.ac: Compute the required llvm static libraries only once

In order to determine which static LLVM libraries are needed we pass
a list of components to llvm-config and it generates the list of
library dependencies for us.  The advantage of only calling llvm-config
one time is that it can determine if two components depend on the same
library and then add it to the output list only once.  The old practice
of having each driver call llvm-config to add its own dependencies to
$(LLVM_LIBS) caused many libraries to be added to this variable multiple
times.