[SCons] Make C++14 minimum requirement

The minimum language standard to use Boost.Math will be C++14
starting in July 2023

CONTRIBUTING.md

@@ -76,8 +76,8 @@
 * Class names use `InitialCapsNames`
 * Methods use `camelCaseNames`
 * Do not indent the contents of namespaces
-* Code should follow the C++11 standard, with minimum required compiler versions
-  GCC 4.8, Clang 3.4, MSVC 14.0 (2015) and Intel 15.0.
+* Code should follow the C++14 standard, with minimum required compiler versions
+  GCC 6.1, Clang 3.4, MSVC 14.1 (2017) and Intel 17.0.
 * Avoid manual memory management (that is, `new` and `delete`), preferring to use
   standard library containers, as well as `std::unique_ptr` and
   `std::shared_ptr` when dynamic allocation is required.

SConstruct

@@ -196,11 +196,11 @@ config_options = [
     Option(
         "cxx_flags",
         """Compiler flags passed to the C++ compiler only. Separate multiple
-           options with spaces, for example, "cxx_flags='-g -Wextra -O3 --std=c++11'"
+           options with spaces, for example, "cxx_flags='-g -Wextra -O3 --std=c++14'"
            """,
         {
             "cl": "/EHsc",
-            "default": "-std=c++11"
+            "default": "-std=c++14"
         }),
     Option(
         "CC",

interfaces/python_sdist/setup.py

@@ -146,7 +146,7 @@ def create_config(key, value):
         create_config("SOLARIS", 0)
 
     if sys.platform != "win32":
-        extra_compile_flags = ["-std=c++11", "-g0"]
+        extra_compile_flags = ["-std=c++14", "-g0"]
         sundials_configh = {
             "SUNDIALS_USE_GENERIC_MATH": "#define SUNDIALS_USE_GENERIC_MATH 1",
             "SUNDIALS_BLAS_LAPACK": "/* #undef SUNDIALS_BLAS_LAPACK */"