Cmake: Require at least CMake 2.8.

CMake 2.6 is already a bit old. Many bugs have been fixed since
its release. We use 2.8 in our company and we have no intention
of polluting our environment with old software, so 2.6 would
not be tested. This shouldn't be a problem since all one need
to build CMake from source is C and C++ compiler.

CMakeLists.txt

@@ -38,7 +38,7 @@
 # To check:
 # (From Daniel Stenberg) The cmake build selected to run gcc with -fPIC on my box while the plain configure script did not.
 # (From Daniel Stenberg) The gcc command line use neither -g nor any -O options. As a developer, I also treasure our configure scripts's --enable-debug option that sets a long range of "picky" compiler options.
-cmake_minimum_required(VERSION 2.6.2 FATAL_ERROR)
+cmake_minimum_required(VERSION 2.8 FATAL_ERROR)
 set(CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/CMake;${CMAKE_MODULE_PATH}")
 include(Utilities)
 
@@ -250,7 +250,7 @@ option(CURL_ZLIB "Set to ON to enable building cURL with zlib support." ON)
 set(HAVE_LIBZ OFF)
 set(HAVE_ZLIB_H OFF)
 set(HAVE_ZLIB OFF)
-if(CURL_ZLIB)  # AND CURL_CONFIG_HAS_BEEN_RUN_BEFORE
+if(CURL_ZLIB)
   find_package(ZLIB QUIET)
   if(ZLIB_FOUND)
     set(HAVE_ZLIB_H ON)
@@ -262,13 +262,13 @@ endif()
 
 option(CMAKE_USE_OPENSSL "Use OpenSSL code. Experimental" ON)
 mark_as_advanced(CMAKE_USE_OPENSSL)
+
+set(USE_SSLEAY OFF)
+set(USE_OPENSSL OFF)
+set(HAVE_LIBCRYPTO OFF)
+set(HAVE_LIBSSL OFF)
+
 if(CMAKE_USE_OPENSSL)
-
-  set(USE_SSLEAY OFF)
-  set(USE_OPENSSL OFF)
-  set(HAVE_LIBCRYPTO OFF)
-  set(HAVE_LIBSSL OFF)
-
   find_package(OpenSSL)
   if(OPENSSL_FOUND)
     list(APPEND CURL_LIBS ${OPENSSL_LIBRARIES})