Where to put (or find) #include files for Arduino IDE?

A while ago I put in a lot of effort trying to get a 20x4 LCD display work thru its I2C interface. Found lots of example programs, but couldn’t get them to work, the system couldn’t find the LiquidCrystal_I2C.h file.

I went a little nutz and started putting that file in every directory I could think of. Then later found some other errors in the program, fixed them, and voila it all worked.

Now I can’t figure out which copy of LiquidCrystal_I2C.h the program is using when it encounters the #include <LiquidCrystal_I2C.h> command.

How do I find out what directory the IDE is looking into, to find its #include files?

See here:

If you use one of the newer IDE versions, and turn on "verbose" compilation in the preferences, you'll see (should see?) a report at the bottom of the compile:

"/Applications/arduino/Arduino-1.6.12.app/Contents/Java/portable/packages/arduino/tools/arm-none-eabi-gcc/4.8.3-2014q1/bin/arm-none-eabi-objcopy" -O binary  "/var/folders/jz/5yb8f2hr8xjcpf0059bsfz4r0000gn/T/arduino_build_931376/SerialDisplay.ino.elf" "/var/folders/jz/5yb8f2hr8xjcpf0059bsfz4r0000gn/T/arduino_build_931376/SerialDisplay.ino.bin"
[color=brown]Using library LiquidCrystal at version 1.0.5 in folder: /Applications/arduino/Arduino-1.6.12.app/Contents/Java/libraries/LiquidCrystal 
Sketch uses 23,332 bytes (4%) of program storage space. Maximum is 524,288 bytes