Fix Windows warnings.

Cmake issued a warning when the variable is in quotation marks
in an if statement.

Visual Studio upgrades constants to int and issues a truncation
warning, so inserted a cast.

Change-Id: I60cdcb5c2565cd5e97f80b9c2ff1e6abc32b1deb
Tested: Builds without warning on VS2015.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 1af3d33..b3fa774 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -119,7 +119,7 @@
   set(CMAKE_CXX_FLAGS
     "${CMAKE_CXX_FLAGS} -fsigned-char")
 
-elseif("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
+elseif(${CMAKE_CXX_COMPILER_ID} MATCHES "Clang")
   set(CMAKE_CXX_FLAGS
       "${CMAKE_CXX_FLAGS} -std=c++0x -stdlib=libc++ -Wall -pedantic -Werror \
                           -Wextra")