CMake: Use only CMAKE_INSTALL_BINDIR instead of DIR_BIN

This way it always shows up in ccmake, even if not defined.

A string is used instead of path because I personally think it makes more
sense in the following use-cases:

STRING:
-DCMAKE_INSTALL_PREFIX=/home/user -DCMAKE_INSTALL_BINDIR=bins
installs everything to /home/user/{lib,etc,share,(...)} and executables to
${CMAKE_INSTALL_PREFIX}/bins

-DCMAKE_INSTALL_PREFIX=/home/user -DCMAKE_INSTALL_BINDIR=/opt/bin
everything to /home/user/{lib,etc,share,(...)} and executables to
/opt/bin

PATH:
-DCMAKE_INSTALL_PREFIX=/home/user -DCMAKE_INSTALL_BINDIR=bins
everything to /home/user/{lib,etc,share,(...)} and executables to
$(pwd)/bins (!)
-DCMAKE_INSTALL_PREFIX=/home/user -DCMAKE_INSTALL_BINDIR=/opt/bin
same as STRING

CMakeLists.txt

@@ -34,12 +34,10 @@ set (DIR_LIBRARY ${DIR_PREFIX}/${CMAKE_SHARED_LIBRARY_PREFIX})
 set (DIR_LIBRARY_STATIC ${DIR_PREFIX}/${CMAKE_STATIC_LIBRARY_PREFIX})
 set (DIR_INCLUDE ${DIR_PREFIX}/include)
 set (DIR_SHARE ${DIR_PREFIX}/share)
-set (DIR_BIN ${DIR_PREFIX}/bin)
 set (DIR_ETC ${DIR_PREFIX}/etc)
 
-if (DEFINED CMAKE_INSTALL_BINDIR)
-	set (DIR_BIN ${CMAKE_INSTALL_BINDIR})
-endif (DEFINED CMAKE_INSTALL_BINDIR)
+set (CMAKE_INSTALL_BINDIR bin
+     CACHE STRING "Install location of executables")
 
 if (DEFINED CMAKE_INSTALL_LIBDIR)
 	set (DIR_LIBRARY ${CMAKE_INSTALL_LIBDIR})

src/CMakeLists.txt

@@ -108,7 +108,7 @@ install(
 	TARGETS
 		${UCHARDET_TARGET}
         RUNTIME DESTINATION
-                ${DIR_BIN}
+                ${CMAKE_INSTALL_BINDIR}
 	ARCHIVE DESTINATION
 		${DIR_LIBRARY}
 )

src/tools/CMakeLists.txt

@@ -18,6 +18,6 @@ install(
 		uchardet
 	RUNTIME
 	DESTINATION
-		${DIR_BIN}
+		${CMAKE_INSTALL_BINDIR}
 )