build: Fix -Wconversion warnings for fpclassify et al

closes #8274

The parent commit tries a different approach, but that fails on Apple
Clang version:
    Apple LLVM version 10.0.0 (clang-1000.11.45.5)
    Target: x86_64-apple-darwin17.7.0
which somehow compiles the check_c_source_compiles() check, but then
complains during later compilation that __fpclassify is not defined
(regardless of "#include <math.h>").

CMakeLists.txt

@@ -258,19 +258,6 @@ int main(void)
 }
 " HAVE_BUILTIN_ADD_OVERFLOW)
 
-set(CMAKE_REQUIRED_LIBRARIES m)
-check_c_source_compiles("
-#include <math.h>
-int main(void)
-{
-  double d = 1.0;
-  int ok = (FP_NORMAL == __fpclassify(d) && !__isnan(d) && !__isinf(d));
-  return ok ? 0 : 1;
-}
-" HAVE___FPCLASSIFY)
-unset(CMAKE_REQUIRED_LIBRARIES)
-
-
 if(MSVC)
   # XXX: /W4 gives too many warnings. #3241
   add_definitions(/W3 -D_CRT_SECURE_NO_WARNINGS -D_CRT_NONSTDC_NO_DEPRECATE)

config/config.h.in

@@ -71,6 +71,5 @@
 
 #cmakedefine HAVE_EXECINFO_BACKTRACE
 #cmakedefine HAVE_BUILTIN_ADD_OVERFLOW
-#cmakedefine HAVE___FPCLASSIFY
 
 #endif  // AUTO_CONFIG_H

src/nvim/eval/encode.c

@@ -19,6 +19,7 @@
 #include "nvim/eval/typval.h"
 #include "nvim/garray.h"
 #include "nvim/mbyte.h"
+#include "nvim/math.h"
 #include "nvim/message.h"
 #include "nvim/memory.h"
 #include "nvim/charset.h"  // vim_isprintc()

src/nvim/macros.h

@@ -1,10 +1,6 @@
 #ifndef NVIM_MACROS_H
 #define NVIM_MACROS_H
 
-#include <math.h>
-
-#include "auto/config.h"
-
 // EXTERN is only defined in main.c. That's where global variables are
 // actually defined and initialized.
 #ifndef EXTERN
@@ -23,23 +19,6 @@
 # define MAX(X, Y) ((X) > (Y) ? (X) : (Y))
 #endif
 
-#if defined(__clang__) && __clang__ == 1 && __clang_major__ >= 6 \
-    && defined(HAVE___FPCLASSIFY)
-// Workaround glibc + Clang 6+ bug. #8274
-// https://bugzilla.redhat.com/show_bug.cgi?id=1472437
-# define xfpclassify __fpclassify
-# define xisnan __isnan
-# define xisinf __isinf
-#elif defined(__MINGW32__)
-// Workaround mingw warning. #7863
-# define xfpclassify __fpclassify
-# define xisnan _isnan
-#else
-# define xfpclassify fpclassify
-# define xisnan isnan
-# define xisinf isinf
-#endif
-
 /// String with length
 ///
 /// For use in functions which accept (char *s, size_t len) pair in arguments.

src/nvim/math.c

@@ -0,0 +1,42 @@
+// This is an open source non-commercial project. Dear PVS-Studio, please check
+// it. PVS-Studio Static Code Analyzer for C, C++ and C#: http://www.viva64.com
+
+#include <math.h>
+
+#include "nvim/math.h"
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "math.c.generated.h"
+#endif
+
+#if defined(__clang__) && __clang__ == 1 && __clang_major__ >= 6
+// Workaround glibc + Clang 6+ bug. #8274
+// https://bugzilla.redhat.com/show_bug.cgi?id=1472437
+# pragma clang diagnostic push
+# pragma clang diagnostic ignored "-Wconversion"
+#endif
+int xfpclassify(double d)
+{
+#if defined(__MINGW32__)
+  // Workaround mingw warning. #7863
+  return __fpclassify(d);
+#else
+  return fpclassify(d);
+#endif
+}
+int xisinf(double d)
+{
+  return isinf(d);
+}
+int xisnan(double d)
+{
+#if defined(__MINGW32__)
+  // Workaround mingw warning. #7863
+  return _isnan(d);
+#else
+  return isnan(d);
+#endif
+}
+#if defined(__clang__) && __clang__ == 1 && __clang_major__ >= 6
+# pragma clang diagnostic pop
+#endif

src/nvim/math.h

@@ -0,0 +1,7 @@
+#ifndef NVIM_MATH_H
+#define NVIM_MATH_H
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "math.h.generated.h"
+#endif
+#endif  // NVIM_MATH_H

src/nvim/strings.c

@@ -27,6 +27,7 @@
 #include "nvim/func_attr.h"
 #include "nvim/getchar.h"
 #include "nvim/mark.h"
+#include "nvim/math.h"
 #include "nvim/mbyte.h"
 #include "nvim/memfile.h"
 #include "nvim/memline.h"