Factor platform specific makefile directives out into their own makefile

fragments.  This is gross, but having tons of confusing conditionals all
throughout the build system seems worst.

Credits got to Casey Carter for the idea.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@3705 91177308-0d34-0410-b5e6-96231b3b80d8

Makefile.Linux

@@ -0,0 +1,18 @@
+#===-- Makefile.Linux - Linux specific makefile options ---*- makefile -*--====#
+#
+# This file is included by all other files, through Makefile.common, but only
+# when compiling on a Linux based platform.
+#
+#===-----------------------------------------------------------------------====#
+
+# MakeSharedObjectOption - This option is passed to the linker by
+# Makefile.common when compiling a shared object.
+#
+MakeSharedObjectOption := -shared
+
+# ExportSymbols - This is used by tools that need to make sure that all of their
+# symbols are exported for use at runtime by dlsym.
+#
+ExportSymbols := -Wl,--export-dynamic
+
+