libcpprestsdk: fix building as a static library (#1344)

On Unix, try to use pkg-config to find OpenSSL. This will automatically
find any dependent libraries and put them in the correct order for
linking. If pkg-config is not available or system is not UNIX, fallback
on current mechanism

Signed-off-by: Adam Duskett <aduskett@gmail.com>
[Retrieved (and slightly updated) from:
https://git.buildroot.net/buildroot/tree/package/libcpprestsdk/0001-libcpprestsdk-fix-building-as-a-static-library.patch]
Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>

Author: ffontaine

Release/cmake/cpprest_find_openssl.cmake

@@ -41,7 +41,15 @@ function(cpprest_find_openssl)
       # This should prevent linking against the system provided 0.9.8y
       set(_OPENSSL_VERSION "")
     endif()
-    find_package(OpenSSL 1.0.0 REQUIRED)
+    if(UNIX)
+      find_package(PkgConfig)
+      pkg_search_module(OPENSSL openssl)
+    endif()
+    if(OPENSSL_FOUND)
+      target_link_libraries(cpprest PRIVATE ${OPENSSL_LDFLAGS})
+    else()
+      find_package(OpenSSL 1.0.0 REQUIRED)
+    endif()
 
     INCLUDE(CheckCXXSourceCompiles)
     set(CMAKE_REQUIRED_INCLUDES "${OPENSSL_INCLUDE_DIR}")
@@ -67,4 +75,4 @@ function(cpprest_find_openssl)
     # libressl doesn't ship with the cleanup method being used in ws_client_wspp
     target_compile_definitions(cpprestsdk_openssl_internal INTERFACE -DCPPREST_NO_SSL_LEAK_SUPPRESS)
   endif()
-endfunction()
+endfunction()