title: make extern variable globally visible across all projects in sln file
body: <p>project_1.vcproj contains:
  header_1.h has "extern int flag;" is declared.
  source_1.c has "int flag = 0" initialized</p>
  
  <p>project_2.vcproj contains:
  source_2.c has a function which modifies the value of flag.</p>
  
  <pre><code>#include "header_1.h"
   void setFlag() {
      flag = 1;
  }
  </code></pre>
  
  <p>When I try to compile this I get link errors:
  error LNK2001: unresolved external symbol "int flag"</p>
  
  <p>What I am trying to achieve is I want the value of flag to be visible globally. 
  Because after the flag is set to 1, source_1.c uses this information do other things. </p>
  
  <p>Please let me know how to achieve me. Thanks.</p>