build/CMake: use CMake-2.8.7-compatible list-check (#8220)

Tested with:
    make CMAKE_EXTRA_FLAGS='-DLANGUAGES="en_GB;ja;sk"'

closes #8219

CMakeLists.txt

@@ -5,10 +5,6 @@ if(POLICY CMP0059)
   cmake_policy(SET CMP0059 OLD)  # Needed until cmake 2.8.12. #4389
 endif()
 
-if(POLICY CMP0057)
-  cmake_policy(SET CMP0057 NEW)
-endif()
-
 # Point CMake at any custom modules we may ship
 list(APPEND CMAKE_MODULE_PATH "${PROJECT_SOURCE_DIR}/cmake")
 

src/nvim/po/CMakeLists.txt

@@ -139,17 +139,17 @@ if(HAVE_WORKING_LIBINTL AND GETTEXT_FOUND AND XGETTEXT_PRG AND ICONV_PRG)
   endmacro()
 
   # Create some translations from others.
-  if("ja" IN_LIST LANGUAGES)
+  if(";${LANGUAGES};" MATCHES ";ja;")
     BuildPoIconv(ja utf-8 euc-jp)
     BuildMo(ja.euc-jp)
   endif()
 
-  if("cs" IN_LIST LANGUAGES)
+  if(";${LANGUAGES};" MATCHES ";cs;")
     BuildPoIconv(cs ISO-8859-2 cp1250)
     BuildMo(cs.cp1250)
   endif()
 
-  if("sk" IN_LIST LANGUAGES)
+  if(";${LANGUAGES};" MATCHES ";sk;")
     BuildPoIconv(sk ISO-8859-2 cp1250)
     BuildMo(sk.cp1250)
   endif()
@@ -159,7 +159,7 @@ if(HAVE_WORKING_LIBINTL AND GETTEXT_FOUND AND XGETTEXT_PRG AND ICONV_PRG)
         ${CMAKE_CURRENT_SOURCE_DIR}/no.po ${CMAKE_CURRENT_SOURCE_DIR}/nb.po
     DEPENDS no.po)
   list(APPEND UPDATE_PO_TARGETS update-po-nb)
-  if("nb" IN_LIST LANGUAGES)
+  if(";${LANGUAGES};" MATCHES ";no;")
     CheckPo(nb)
     BuildMo(nb)
   endif()