Add defaults api to openssl build

Build time defaults aren't great for windows, in which various macros
(like OPENSSLDIR) are selected at build time, but may be selected
differently at install time.  Add an internal defaults api to return the
build time constants on unix systems, but instead query registry keys
for the form:
HLKM\SOFTWARE\OpenSSL-{version}-{wininstallcontext}
Such that each built version of openssl may maintain its own set of
registry keys to identify these locations, and be set administratiely as
appropriate at install or run time

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24450)

1 files changed, 22 insertions, 0 deletions

diff --git a/NOTES-WINDOWS.md b/NOTES-WINDOWS.md
index f4573fd430..e218f28bee 100644
--- a/NOTES-WINDOWS.md
+++ b/NOTES-WINDOWS.md
@@ -125,6 +125,28 @@ Administrator" before running `nmake install`.  The other solution
 is, of course, to choose a different set of directories by using
 `--prefix` and `--openssldir` when configuring.
 
+Note that, on Windows platforms (both 32 and 64 bit), the above build time
+defaults can be overridden by registry keys.  This is done because it is common
+practice for windows based installers to allow users to place the installation
+tree at various locations not defined at build time.  The following keys:
+
+    `\\HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\OpenSSL\OPENSSLDIR`
+    `\\HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\OpenSSL\ENGINESDIR`
+    `\\HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\OpenSSL\MODULESDIR`
+
+Can be administratively set, and openssl will take the paths found there as the
+values for OPENSSLDIR, ENGINESDIR and MODULESDIR respectively.  If unset, the
+build time defaults will be used.
+
+To enable the reading of registry keys from windows builds, add
+`-DWININSTALLCONTEXT=<string>`to the Configure command line.  Without setting
+this, the library defaults back to reporing the build time defaults without
+checking the registry.  Note that if you wish to have a private set of registry
+keys for your application, you should set `OPENSSL_VERSION` to a unique value
+
+Note the installer available at <https://github.com/openssl/installer> will set
+these keys when the installer is run.
+
 Special notes for Universal Windows Platform builds, aka `VC-*-UWP`
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