Moved default build and C++ CMake options to a separate file.

cmake/configure_cpp_options.cmake

+
+#
+# Set build options and C++ standards and options
+#
+# This file sets up
+#
+#   CMAKE_BUILD_TYPE
+#   CMAKE_CXX_STANDARD
+#   CMAKE_CXX_EXTENSIONS
+#   CMAKE_CXX_STANDARD_REQUIRED
+#
+# The options can be overridden at configuration time by using, e.g.:
+#    `cmake -DCMAKE_BUILD_TYPE=Debug -DCMAKE_CXX_STANDARD=14 ../GeoModelIO`
+# on the command line.
+#
+
+
+# Set default build options.
+set( CMAKE_BUILD_TYPE "Release" CACHE STRING "CMake build mode to use" )
+set( CMAKE_CXX_STANDARD 17 CACHE STRING "C++ standard used for the build" )
+set( CMAKE_CXX_EXTENSIONS FALSE CACHE BOOL "(Dis)allow using GNU extensions" )
+set( CMAKE_CXX_STANDARD_REQUIRED TRUE CACHE BOOL
+   "Require the specified C++ standard for the build" )