Don't Require Floating Point from_chars() Function

This commit broadens the scope of commit 2ad332e0b (PR #922) to
apply to all compilers/libraries, not just Clang/libc++, which do
not have support for floating-point types in std::from_chars().
While hopefully a transient situation, this enables building the
parameter system with GCC versions prior to GCC 11.  We expect to
require version 11 in the not too distant future, though.  At that
point we should revert this commit.

We use a configure-time feature test of the compiler (CMake command
'try_compile') to detect whether or not the compiler supports
floating-point overloads of std::from_chars() and emit the result to
config.h as the new preprocessor symbol

    HAVE_FLOATING_POINT_FROM_CHARS

We use std::strtod() as the fall-back alternative for floating point
conversion if this symbol is defined to false (zero).

cmake/test/testFloatFromChars.cpp

@@ -0,0 +1,11 @@
+// CMake feature test for floating-point std::from_chars() support
+
+#include <charconv>
+#include <string_view>
+
+int main()
+{
+    const auto s = std::string_view { "2.71828" };
+    auto e = 0.0;
+    std::from_chars(s.data(), s.data() + s.size(), e);
+}