Fix #98 - Build broken using CMake 2.8.8.

As per the CMake docs, LANGUAGE is only to be set for files that are actually compiled. In versions prior to 2.8.7, setting it globally worked fine nevertheless.

CMakeLists.txt

@@ -224,10 +224,13 @@ set(LDC_SOURCE_FILES
     ${GEN_SRC}
     ${IR_SRC}
 )
-set_source_files_properties(
-    ${LDC_SOURCE_FILES}    PROPERTIES
-    LANGUAGE CXX
-)
+
+# DMD source files have a .c extension, but are actually C++ code.
+foreach(file ${LDC_SOURCE_FILES})
+    if(file MATCHES ".*\\.c$")
+        set_source_files_properties(${file} PROPERTIES LANGUAGE CXX)
+    endif()
+endforeach()
 
 #
 # Includes, defines.