Configure clang-format for CMake source tree

After discussion among developers we settled on the Mozilla style
with a few tweaks:

* Do not align operator arguments.

* Do not always break after a function return type.

* Limit to 79 columns instead of 80 to fit edge cases
  better in 80-column terminals as CMake has always done.

* Format for C++98 instead of C++11 because CMake is written
  in the former language.

Co-Author: Daniel Pfeifer <[email protected]>

.clang-format

@@ -0,0 +1,9 @@
+---
+# This configuration requires clang-format 3.8 or higher.
+BasedOnStyle: Mozilla
+AlignOperands: false
+AlwaysBreakAfterReturnType: None
+AlwaysBreakAfterDefinitionReturnType: None
+ColumnLimit: 79
+Standard: Cpp03
+...