Build: make it possible to assign macro definitions for specific outputs

Sometimes, some specific program or object file might need an extra
macro definition of its own.  This allows that to be easily done.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/7553)

Configurations/README

diff --git a/Configurations/README b/Configurations/README
index 9fd4922..1c67f75 100644
--- a/Configurations/README
+++ b/Configurations/README
@@ -467,6 +467,10 @@
 
     INCLUDE[foo]=include
 
+It's also possible to specify C macros that should be defined:
+
+    DEFINE[foo]=FOO BAR=1
+
 In some cases, one might want to generate some source files from
 others, that's done as follows: