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>").

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