clang does not have syslimits.h, it is gcc specific
and it is useless anyway since <limits.h> is already included
so just remove it

Cannot use PFNRT here on clang because of the exception specification

--- a/src/VBox/ExtPacks/VBoxDTrace/include/VBoxDTraceLibCWrappers.h
+++ b/src/VBox/ExtPacks/VBoxDTrace/include/VBoxDTraceLibCWrappers.h
@@ -34,8 +34,6 @@
 # include <limits.h>        /* Workaround for syslimit.h bug in gcc 4.8.3 on gentoo. */
 # ifdef RT_OS_DARWIN
 #  include <sys/syslimits.h> /* PATH_MAX */
-# elif !defined(RT_OS_SOLARIS) && !defined(RT_OS_FREEBSD)
-#  include <syslimits.h>    /* PATH_MAX */
 # endif
 # include <libgen.h>        /* basename */
 # include <unistd.h>
--- a/src/VBox/Devices/Graphics/DevVGA-SVGA3d-glLdr.h
+++ b/src/VBox/Devices/Graphics/DevVGA-SVGA3d-glLdr.h
@@ -97,7 +97,11 @@
 /** Load OpenGL library and initialize function pointers. */
 int glLdrInit(PPDMDEVINS pDevIns);
 /** Resolve an OpenGL function name. */
+#ifdef __clang__
+void* glLdrGetProcAddress(const char *pszSymbol);
+#else	// !__clang__
 PFNRT glLdrGetProcAddress(const char *pszSymbol);
+#endif	// !__clang__
 /** Get pointers to extension function. They are available on Windows only when OpenGL context is set. */
 int glLdrGetExtFunctions(PPDMDEVINS pDevIns);
 
--- a/src/VBox/Devices/Graphics/DevVGA-SVGA3d-glLdr.cpp
+++ b/src/VBox/Devices/Graphics/DevVGA-SVGA3d-glLdr.cpp
@@ -35,6 +35,10 @@
 #include <iprt/ldr.h>
 #include <iprt/log.h>
 
+#ifdef __clang__
+# define PFNRT void*
+#endif
+
 #ifdef RT_OS_WINDOWS
 # define OGLGETPROCADDRESS MyWinGetProcAddress
 DECLINLINE(PFNRT) MyWinGetProcAddress(const char *pszSymbol)