[build] fix when openssl is static build fail

[lilinxiong]

In actual usage, I experienced a problem where I set(OpenSSL_DIR /path/lib/cmake/OpenSSL) and then integrated the SRT project into my own by using add_subdirectory. The first CMake configuration would succeed, but the second configuration attempt would fail.

The details are as follows:

set(OpenSSL_DIR ${THIRD_PARTY_DIR}/binaries/openssl-android/${ANDROID_ABI}/lib/cmake/OpenSSL)
message("OpenSSL_DIR:${OpenSSL_DIR}")
find_package(OpenSSL REQUIRED CONFIG)

if (OpenSSL_FOUND)
    message("OpenSSL_FOUND = ${OpenSSL_FOUND}")
    message("OPENSSL_INCLUDE_DIR = ${OPENSSL_INCLUDE_DIR}")
    message("OPENSSL_LIBRARY_DIR = ${OPENSSL_LIBRARY_DIR}")
    message("OPENSSL_LIBRARIES = ${OPENSSL_LIBRARIES}")
endif ()

message("add_subdirectory: srt:${SRT_DIR} ......")
set(ENABLE_APPS OFF)
set(ENABLE_SHARED OFF)
set(USE_OPENSSL_PC OFF)
set(OPENSSL_USE_STATIC_LIBS ON)
add_subdirectory(${SRT_DIR} ./srt-bin)

message("add_subdirectory: srt:${SRT_DIR} done!")

It seems to be ok, but when I do the first cmake config, and the second cmake config with caching, I get different results.

In the above message, OPENSSL_INCLUDE_DIR has a value the first time, and is empty the second time.

After cmake config and troubleshooting again and again, I finally found that: CMakeLists.txt#L175-OPENSSL_USE_STATIC_LIBS will cause cmake config to fail.

The reason is that there will be the same variables in OpenSSLConfig.cmake.

....
if(OPENSSL_USE_STATIC_LIBS)
  set(_ossl_use_static_libs True)
elseif(DEFINED OPENSSL_USE_STATIC_LIBS)
  # We know OPENSSL_USE_STATIC_LIBS is defined and False
  if(_ossl_use_static_libs)
    # OPENSSL_USE_STATIC_LIBS is explicitly false, indicating that shared libraries are
    # required.  However, _ossl_use_static_libs indicates that no shared libraries are
    # available.  The best course of action is to simply return and leave it to CMake to
    # use another OpenSSL config.
    unset(_ossl_use_static_libs)
    unset(CMAKE_IMPORT_FILE_VERSION)
    return()
  endif()
endif()
....

Because there is no cache when cmake config is run for the first time, it works fine, but the second time, there is a cache, which causes failure.

We should not define the same variables as those in config.cmake generated by make install, we should name them differently.