Build from release tarballs without git or a vcpkg clone.

CMakeLists.txt runs `git rev-parse` in sdk/ for the version string, and
clones vcpkg into ../vcpkg whenever that directory does not exist. A
tarball build has no git checkout, and the clone needs the network.
Take the SDK hash from a cache variable instead, and clone vcpkg only
when VCPKG_ROOT is set explicitly; empty selects the system libraries.

diff --git a/CMakeLists.txt b/CMakeLists.txt
index d368673..caab827 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,13 +1,7 @@
 # CMakeLists.txt file to build the MEGAcmd
 cmake_minimum_required(VERSION 3.16)
 
-find_package(Git REQUIRED)
-execute_process(
-    COMMAND ${GIT_EXECUTABLE} rev-parse --short HEAD
-    WORKING_DIRECTORY ${CMAKE_CURRENT_LIST_DIR}/sdk
-    OUTPUT_VARIABLE SDK_COMMIT_HASH
-    OUTPUT_STRIP_TRAILING_WHITESPACE
-)
+set(SDK_COMMIT_HASH "" CACHE STRING "Short commit hash of the bundled SDK")
 
 set(MEGACMD_MAJOR_VERSION 2)
 set(MEGACMD_MINOR_VERSION 7)
@@ -26,7 +20,7 @@ set(QT_CREATOR_SKIP_VCPKG_SETUP TRUE CACHE BOOL "")
 list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_LIST_DIR}/build/cmake/modules) # Modules from MEGAcmd
 list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_LIST_DIR}/sdk/cmake/modules) # Modules from MEGAsdk
 
-set(VCPKG_ROOT "${CMAKE_CURRENT_LIST_DIR}/../vcpkg" CACHE PATH "If set, it will build and use the VCPKG packages defined in the manifest file")
+set(VCPKG_ROOT "" CACHE PATH "If set, it will build and use the VCPKG packages defined in the manifest file")
 
 include(detect_host_architecture)
 
@@ -39,7 +33,7 @@ execute_process(
 )
 endif()
 
-if((NOT WIN32 OR BASH_VERSION_RESULT EQUAL 0) AND NOT EXISTS ${VCPKG_ROOT})
+if(VCPKG_ROOT AND (NOT WIN32 OR BASH_VERSION_RESULT EQUAL 0) AND NOT EXISTS ${VCPKG_ROOT})
     message(STATUS "vcpkg will be cloned into ${VCPKG_ROOT}")
     execute_process(
         #TODO: have the same for windows ... or at least check if bash is available