Optionally locate boost via environment variable BOOST_ROOT

This was already the case for our autotools based build
and simplifies the build setup for our Windows build system.

CMakeLists.txt

@@ -131,7 +131,6 @@ IF (WIN32)
   SET(CMAKE_FRAMEWORK_PATH "${CMAKE_FRAMEWORK_PATH_TMP}")
   SET(PKG_CONFIG_EXECUTABLE ${CMAKE_PREFIX_PATH}/gnome/bin/pkg-config.exe)
   SET(SWIG_EXECUTABLE ${CMAKE_PREFIX_PATH}/swig/swig.exe)
-  SET(BOOST_ROOT ${CMAKE_PREFIX_PATH}/boost)
   SET(REGEX_INCLUDE_PATH ${CMAKE_PREFIX_PATH}/regex/include)
   SET(REGEX_LDFLAGS "-L${CMAKE_PREFIX_PATH}/regex/lib -lregex")
   SET(LIBXSLT_INCLUDE_DIR ${CMAKE_PREFIX_PATH}/libxslt/include)
@@ -420,6 +419,9 @@ ENDIF()
 SET (Boost_USE_MULTITHREADED ON)
 SET (Boost_FIND_QUIETLY ON)
 
+IF (NOT DEFINED ${BOOST_ROOT})
+  SET(BOOST_ROOT $ENV{BOOST_ROOT})
+ENDIF()
 FIND_PACKAGE (Boost 1.54.0 REQUIRED COMPONENTS date_time regex)
 
 IF (Boost_FOUND)