This logic mistakes "clang" for "CL.EXE". Let's just cut out the Windows
Index: GCC/CMakeLists.txt
===================================================================
--- GCC/CMakeLists.txt.orig
@@ -23,18 +23,6 @@ IF(APPLE)
SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${CMAKE_ANSI_CFLAGS}")
-IF(CMAKE_C_COMPILER MATCHES "cl")
- STRING(REGEX REPLACE "/W([0-4])?" "" CMAKE_C_FLAGS "${CMAKE_C_FLAGS}")
- SET(CMAKE_CXX_WARNING_LEVEL 0)
- SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} /W0")
-ELSE(CMAKE_C_COMPILER MATCHES "cl")
- IF(CMAKE_C_COMPILER MATCHES "bcc32")
- SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -w-")
- ELSE(CMAKE_C_COMPILER MATCHES "bcc32")
- SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -w")
- ENDIF(CMAKE_C_COMPILER MATCHES "bcc32")
-ENDIF(CMAKE_C_COMPILER MATCHES "cl")
ADD_DEFINITIONS(-DALMOST_STDC)
SUBDIRS(config_cmake libiberty libdecnumber libcpp gcc)