gtk/fallback-c89.c: Add C89 fallback for trunc()

Check for trunc() during configure and provide a fallback implementation
for it if it is not found.

config.h.win32.in

@@ -209,6 +209,11 @@
 /* Define to 1 if you have the <sys/types.h> header file. */
 #define HAVE_SYS_TYPES_H 1
 
+/* Define to 1 if trunc() is available */
+#if !defined (_MSC_VER) || (_MSC_VER >= 1800)
+# define HAVE_TRUNC 1
+#endif
+
 /* Define to 1 if you have the <unistd.h> header file. */
 #ifndef _MSC_VER
 #define HAVE_UNISTD_H 1

configure.ac

@@ -876,9 +876,10 @@ AC_CHECK_FUNCS(mallinfo)
 AC_CHECK_FUNCS(getresuid)
 AC_TYPE_UID_T
 
-# Check for round(), rint(), isnan(), isinf() and nearbyint()
+# Check for round(), rint(), isnan() and isinf()
+# Check for log2(), exp2(), nearbyint() and trunc()
 AC_CHECK_LIB(m,round,,)
-AC_CHECK_FUNCS(round rint nearbyint sincos exp2 log2)
+AC_CHECK_FUNCS(round rint nearbyint sincos exp2 log2 trunc)
 AC_CHECK_DECLS([isnan, isinf], [], [], [[#include <math.h>]])
 
 # Checks for gdkspawn

gtk/fallback-c89.c

@@ -106,3 +106,11 @@ exp2 (double x)
   return pow (2.0, x);
 }
 #endif
+
+#ifndef HAVE_TRUNC
+static inline double
+trunc (double x)
+{
+  return (x > 0 ? floor (x) : ceil (x));
+}
+#endif