Don't duplicate ceiling() for every POSIX platform.

src/platform-posix.cc

Index: src/platform-posix.cc
diff --git a/src/platform-posix.cc b/src/platform-posix.cc
index 804b38bd5e8f38ac05b6adc1cfc8e992aecedc25..9bc3193631aab80a0b71e799259c2c35b1d19e65 100644
--- a/src/platform-posix.cc
+++ b/src/platform-posix.cc
@@ -225,6 +225,12 @@ void OS::DebugBreak() {
 // ----------------------------------------------------------------------------
 // Math functions
 
+double ceiling(double x) {
+  // Correct buggy 'ceil' on some systems (i.e. FreeBSD, OS X 10.5)
+  return (-1.0 < x && x < 0.0) ? -0.0 : ceil(x);
+}
+
+
 double modulo(double x, double y) {
   return fmod(x, y);
 }