Prevent use of -Ofast optimization in gcc.

The C version of Astronomy Engine does not work correctly
when gcc "fast math" optimizations are enabled.
The problem is that Astronomy Engine uses NAN values to
represent invalid/uninitialized floating point numbers.
The -Ofast option breaks the ability of the runtime to
check for NAN values, resulting in multiple failures
and incorrect behaviors at runtime.

Added a compile-time check for the __FAST_MATH__ preprocessor
symbol, which gcc defines to signal that the optimization
was enabled. If detected, this results in a compiler error
to make it obvious that something is wrong before invalid
code would be executed.

This is not an ideal fix for two reasons:

1. I don't know if this will detect similar problems for
   other compilers than gcc.

2. If individual risky math optimizations are enabled, instead
   of the combination of options included in -Ofast, the
   __FAST_MATH__ preprocessor symbol will not be defined
   and no compiler error will occur. I could not find a
   way to detect individual risky optimizations.

However, this change is much better than nothing, and
hopefully it will prevent most cases of overly-aggressive
optimization.

Author: cosinekitty

Diff for: generate/template/astronomy.c

@@ -32,6 +32,10 @@
 #include <math.h>
 #include "astronomy.h"
 
+#ifdef __FAST_MATH__
+#error Astronomy Engine does not support "fast math" optimization because it causes incorrect behavior. See: https://github.com/cosinekitty/astronomy/issues/245
+#endif
+
 #ifdef __cplusplus
 extern "C" {
 #endif

Diff for: source/c/astronomy.c

@@ -32,6 +32,10 @@
 #include <math.h>
 #include "astronomy.h"
 
+#ifdef __FAST_MATH__
+#error Astronomy Engine does not support "fast math" optimization because it causes incorrect behavior. See: https://github.com/cosinekitty/astronomy/issues/245
+#endif
+
 #ifdef __cplusplus
 extern "C" {
 #endif