[GlobalISel] make library an optional component

Summary:
Mark the LLVMGlobalISel library as optional in
LLVMBuild.txt, since the library is only built
if LLVM_BUILD_GLOBAL_ISEL is set. Without doing
this, llvm-config includes the library in the
list of components regardless of whether it's
built, and then will error out when asked for
the library names/paths.

Reviewers: qcolombet

Subscribers: joker.eph, llvm-commits, vkalintiris

Differential Revision: http://reviews.llvm.org/D16386

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@258379 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/GlobalISel/LLVMBuild.txt

@@ -16,7 +16,7 @@
 ;===------------------------------------------------------------------------===;
 
 [component_0]
-type = Library
+type = OptionalLibrary
 name = GlobalISel
 parent = CodeGen
 required_libraries = Analysis CodeGen Core MC Support Target TransformUtils