fix CMP0053 warning with cmake 3.1

builds/cmake/Modules/FindAsciiDoc.cmake

@@ -6,19 +6,21 @@
 # A2X_EXECUTABLE - the full path to a2x
 # A2X_FOUND - If false, don't attempt to use a2x.
 
+set (PROGRAMFILESX86 "PROGRAMFILES(X86)")
+
 find_program(ASCIIDOC_EXECUTABLE asciidoc asciidoc.py
              PATHS "$ENV{ASCIIDOC_ROOT}"
                    "$ENV{PROGRAMW6432}/asciidoc"
                    "$ENV{PROGRAMFILES}/asciidoc"
-                   "$ENV{PROGRAMFILES(X86)}/asciidoc")
+                   "$ENV{${PROGRAMFILESX86}}/asciidoc")
 
 find_program(A2X_EXECUTABLE a2x
              PATHS "$ENV{ASCIIDOC_ROOT}"
                    "$ENV{PROGRAMW6432}/asciidoc"
                    "$ENV{PROGRAMFILES}/asciidoc"
-                   "$ENV{PROGRAMFILES(X86)}/asciidoc")
+                   "$ENV{${PROGRAMFILESX86}}/asciidoc")
 
 
 include(FindPackageHandleStandardArgs)
 find_package_handle_standard_ARGS(AsciiDoc REQUIRED_VARS ASCIIDOC_EXECUTABLE)
-mark_as_advanced(ASCIIDOC_EXECUTABLE A2X_EXECUTABLE)
\ No newline at end of file
+mark_as_advanced(ASCIIDOC_EXECUTABLE A2X_EXECUTABLE)