Fix missing regex dependency for gtest on QNX

- On QNX gtest requires linking of the system regex library.
- This mirrors gtests' own library build here:
  https://github.com/google/googletest/blob/main/googletest/CMakeLists.txt#L158

Change-Id: I207cf8874f6fce9cfe50faa0414ad70cf355759e

internal/ceres/CMakeLists.txt

diff --git a/internal/ceres/CMakeLists.txt b/internal/ceres/CMakeLists.txt
index 350b3b5..21caa8d 100644
--- a/internal/ceres/CMakeLists.txt
+++ b/internal/ceres/CMakeLists.txt
@@ -427,6 +427,9 @@
   add_library(gtest STATIC gmock_gtest_all.cc gmock_main.cc)
 
   target_include_directories(gtest PRIVATE ${Ceres_SOURCE_DIR}/internal/ceres)
+  if (CMAKE_SYSTEM_NAME MATCHES "QNX")
+    target_link_libraries(gtest PUBLIC regex)
+  endif()
   target_link_libraries(gtest PRIVATE Ceres::ceres gflags)
 
   add_library(test_util STATIC