On Windows, we set INSTALLTOP to default as follows:
VC-WIN32:
PREFIX: %ProgramFiles(x86)%\OpenSSL
OPENSSLDIR: %CommonProgramFiles(x86)%\SSL
VC-WIN64*:
PREFIX: %ProgramW6432%\OpenSSL
OPENSSLDIR: %CommonProgramW6432%\SSL
Should those environment variables be missing, the following is used
as fallback:
PREFIX: %ProgramFiles%\OpenSSL
OPENSSLDIR: %CommonProgramFiles%\SSL
Reviewed-by: Andy Polyakov <appro@openssl.org>