Processing GNU-style "make variables" - separate CPP flags from C flags

C preprocessor flags get separated from C flags, which has the
advantage that we don't get loads of macro definitions and inclusion
directory specs when linking shared libraries, DSOs and programs.

This is a step to add support for "make variables" when configuring.

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5177)
diff --git a/Configurations/README b/Configurations/README
index 9d5dc5c..aa0c5fb 100644
--- a/Configurations/README
+++ b/Configurations/README
@@ -45,6 +45,19 @@
                            Note: if the same feature is both enabled
                            and disabled, disable wins.
 
+        cpp             => The C preprocessor command, normally not
+                           given, as the build file defaults are
+                           usually good enough.
+        cppflags        => The C preprocessor flags.
+        defines         => As an alternative, macro definitions may be
+                           given here instead of in `cppflags'.  If
+                           given here, they MUST be as an array of the
+                           string such as "MACRO=value", or just
+                           "MACRO" for definitions without value.
+        includes        => As an alternative, inclusion directories
+                           may be given here instead of in `cppflags'.
+                           If given here, the MUST be an array of
+                           strings, one directory specification each.
         cc              => The C compiler command, usually one of "cc",
                            "gcc" or "clang".  This command is normally
                            also used to link object files and
@@ -59,11 +72,6 @@
         cxxflags        => Flags that are used at all times when
                            compiling C++ object files.  If unset, it
                            gets the same value as cflags.
-        defines         => As an alternative, macro definitions may be
-                           present here instead of in `cflags'.  If
-                           given here, they MUST be as an array of the
-                           string such as "MACRO=value", or just
-                           "MACRO" for definitions without value.
         shared_cflag    => Extra compilation flags used when
                            compiling for shared libraries, typically
                            something like "-fPIC".