Make pkg-config optional.

Since libunwind, and therefore the CFI tests, are already optional,
might as well make pkg-config optional too.

(I'm not sure whether we actually want to support people using our
development build, but gRPC appear to be trying to do so:
https://github.com/grpc/grpc/issues/17638)

Change-Id: I16b4c53bd8a66933bc19fba29aed0d79ce2670c2
Reviewed-on: https://boringssl-review.googlesource.com/c/34124
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index abd85f5..13ee364 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -40,12 +40,16 @@
 endif()
 
 if(CMAKE_SYSTEM_NAME STREQUAL "Linux")
-  find_package(PkgConfig REQUIRED)
-  pkg_check_modules(LIBUNWIND libunwind-generic)
-  if(LIBUNWIND_FOUND)
-    add_definitions(-DBORINGSSL_HAVE_LIBUNWIND)
+  find_package(PkgConfig QUIET)
+  if (PkgConfig_FOUND)
+    pkg_check_modules(LIBUNWIND libunwind-generic)
+    if(LIBUNWIND_FOUND)
+      add_definitions(-DBORINGSSL_HAVE_LIBUNWIND)
+    else()
+      message("libunwind not found. Disabling unwind tests.")
+    endif()
   else()
-    message("libunwind not found. Disabling unwind tests.")
+    message("pkgconfig not found. Disabling unwind tests.")
   endif()
 endif()