Make it easier to control symbol exports (#7877)

By not hardcoding RDYNAMIC it's possible to control exporting from the
commandline on platforms/compilers that don't support -rdynamic.

Author: sodero

Makefile

@@ -43,7 +43,7 @@ ifdef FILESDIR
     override CPPFLAGS+=-DFILESDIR=\"$(FILESDIR)\"
 endif
 
-RDYNAMIC=-rdynamic
+RDYNAMIC?=-rdynamic
 # Set the CPPCHK_GLIBCXX_DEBUG flag. This flag is not used in release Makefiles.
 # The _GLIBCXX_DEBUG define doesn't work in Cygwin or other Win32 systems.
 ifndef COMSPEC

tools/dmake/dmake.cpp

@@ -621,7 +621,7 @@ int main(int argc, char **argv)
          << "endif\n\n";
 
     // enable backtrac
-    fout << "RDYNAMIC=-rdynamic\n";
+    fout << "RDYNAMIC?=-rdynamic\n";
 
     // The _GLIBCXX_DEBUG doesn't work in cygwin or other Win32 systems.
     fout << "# Set the CPPCHK_GLIBCXX_DEBUG flag. This flag is not used in release Makefiles.\n"