CMake: Allow specifying the mysql location by environment variable.

cmake/macros/FindMySQL.cmake

@@ -94,6 +94,7 @@ find_path(MYSQL_INCLUDE_DIR
     "$ENV{ProgramFiles}/MySQL/*/include"
     "$ENV{SystemDrive}/MySQL/*/include"
     "c:/msys/local/include"
+    "$ENV{MYSQL_ROOT}/include"
   DOC
     "Specify the directory containing mysql.h."
 )
@@ -159,6 +160,7 @@ if( WIN32 )
       "$ENV{ProgramFiles}/MySQL/*/lib/opt"
       "$ENV{SystemDrive}/MySQL/*/lib/opt"
       "c:/msys/local/include"
+      "$ENV{MYSQL_ROOT}/lib"
     DOC "Specify the location of the mysql library here."
   )
 endif( WIN32 )