CMake: Ensure discovered include dirs are considered

...during header checks. Otherwise some following header tests
(incorrectly) fail.

Closes #436

CMakeLists.txt

@@ -355,19 +355,22 @@ if(NOT CURL_DISABLE_LDAP)
     check_include_file_concat("winber.h"  HAVE_WINBER_H)
   endif()
 
+  set(CMAKE_REQUIRED_INCLUDES_BAK ${CMAKE_REQUIRED_INCLUDES})
   set(CMAKE_LDAP_INCLUDE_DIR "" CACHE STRING "Path to LDAP include directory")
   if(CMAKE_LDAP_INCLUDE_DIR)
-    set(CMAKE_REQUIRED_INCLUDES ${CMAKE_LDAP_INCLUDE_DIR})
+    list(APPEND CMAKE_REQUIRED_INCLUDES ${CMAKE_LDAP_INCLUDE_DIR})
   endif()
   check_include_file_concat("ldap.h"           HAVE_LDAP_H)
   check_include_file_concat("lber.h"           HAVE_LBER_H)
 
   if(NOT HAVE_LDAP_H)
     message(STATUS "LDAP_H not found CURL_DISABLE_LDAP set ON")
     set(CURL_DISABLE_LDAP ON CACHE BOOL "" FORCE)
+    set(CMAKE_REQUIRED_INCLUDES ${CMAKE_REQUIRED_INCLUDES_BAK}) #LDAP includes won't be used
   elseif(NOT HAVE_LIBLDAP)
     message(STATUS "LDAP library '${CMAKE_LDAP_LIB}' not found CURL_DISABLE_LDAP set ON")
     set(CURL_DISABLE_LDAP ON CACHE BOOL "" FORCE)
+    set(CMAKE_REQUIRED_INCLUDES ${CMAKE_REQUIRED_INCLUDES_BAK}) #LDAP includes won't be used
   else()
     if(CMAKE_USE_OPENLDAP)
       set(USE_OPENLDAP ON)
@@ -448,6 +451,7 @@ if(CURL_ZLIB)
     set(HAVE_LIBZ ON)
     list(APPEND CURL_LIBS ${ZLIB_LIBRARIES})
     include_directories(${ZLIB_INCLUDE_DIRS})
+    list(APPEND CMAKE_REQUIRED_INCLUDES ${ZLIB_INCLUDE_DIRS})
   endif()
 endif()
 
@@ -463,7 +467,7 @@ if(CMAKE_USE_LIBSSH2)
   if(LIBSSH2_FOUND)
     list(APPEND CURL_LIBS ${LIBSSH2_LIBRARY})
     set(CMAKE_REQUIRED_LIBRARIES ${LIBSSH2_LIBRARY})
-    set(CMAKE_REQUIRED_INCLUDES "${LIBSSH2_INCLUDE_DIR}")
+    list(APPEND CMAKE_REQUIRED_INCLUDES "${LIBSSH2_INCLUDE_DIR}")
     include_directories("${LIBSSH2_INCLUDE_DIR}")
     set(HAVE_LIBSSH2 ON)
     set(USE_LIBSSH2 ON)
@@ -496,7 +500,7 @@ if(CMAKE_USE_GSSAPI)
 
     message(STATUS "Found ${GSS_FLAVOUR} GSSAPI version: \"${GSS_VERSION}\"")
 
-    set(CMAKE_REQUIRED_INCLUDES ${GSS_INCLUDE_DIR})
+    list(APPEND CMAKE_REQUIRED_INCLUDES ${GSS_INCLUDE_DIRECTORIES})
     check_include_file_concat("gssapi/gssapi.h"  HAVE_GSSAPI_GSSAPI_H)
     check_include_file_concat("gssapi/gssapi_generic.h" HAVE_GSSAPI_GSSAPI_GENERIC_H)
     check_include_file_concat("gssapi/gssapi_krb5.h" HAVE_GSSAPI_GSSAPI_KRB5_H)
@@ -532,7 +536,7 @@ if(CMAKE_USE_GSSAPI)
 
     endif()
 
-    include_directories(${GSS_INCLUDE_DIR})
+    include_directories(${GSS_INCLUDE_DIRECTORIES})
     link_directories(${GSS_LINK_DIRECTORIES})
     set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${GSS_COMPILER_FLAGS}")
     set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} ${GSS_LINKER_FLAGS}")
@@ -552,6 +556,7 @@ else()
   unset(USE_UNIX_SOCKETS CACHE)
 endif()
 
+
 # Check for header files
 if(NOT UNIX)
   check_include_file_concat("windows.h"      HAVE_WINDOWS_H)