FindRe2: use re2's package config, whose re2::re2 links abseil

Summary:
`FindRe2.cmake` finds re2 with `find_library()`, which gets only re2 itself. A static re2, such as the one vcpkg builds for eden's Windows build, also needs the abseil libraries it was built against, so linking `edenfs.exe` fails with 48 unresolved `absl::` symbols from `re2.lib`. re2's package config defines `re2::re2` with them, so `FindRe2` now uses that target when re2 has a config, as HHVM's and Velox's builds already do, and still falls back to `find_library()` when it doesn't.

___

landed-with-radar-review

Differential Revision: D128886633

fbshipit-source-id: 1337eb976bc27879fb1badd2bb770a65d5cc47a4

Author: genevievehelsel

build/fbcode_builder/CMake/FindRe2.cmake

@@ -3,7 +3,14 @@
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2.
 
-find_library(RE2_LIBRARY re2)
+# re2's package config, where re2 has one, defines re2::re2 with the abseil
+# libraries that a static re2 needs and a bare find_library() leaves out.
+find_package(re2 CONFIG QUIET)
+if(TARGET re2::re2)
+  set(RE2_LIBRARY re2::re2)
+else()
+  find_library(RE2_LIBRARY re2)
+endif()
 mark_as_advanced(RE2_LIBRARY)
 
 find_path(RE2_INCLUDE_DIR NAMES re2/re2.h)