Possibly incorrect find_package configuration for zlib

[Punchyou]

I get the following output when compiling zynaddsubfx without zlib installed.

-- Found Rtosc Submodule...
-- Found GCC Version >= 4.7
-- Include what you use: disabled (RTOSC_INCLUDE_WHAT_YOU_USE=OFF)
-- Could NOT find OpenGL (missing: OPENGL_opengl_LIBRARY OPENGL_glx_LIBRARY OPENGL_INCLUDE_DIR) 
-- Could NOT find FLTK (missing: FLTK_LIBRARIES FLTK_INCLUDE_DIR FLTK_FLUID_EXECUTABLE) 
-- Checking for module 'ntk'
--   No package 'ntk' found
-- Checking for module 'jack'
--   No package 'jack' found
-- Could NOT find Ruby (missing: RUBY_EXECUTABLE RUBY_INCLUDE_DIR RUBY_LIBRARY) (Required is at least version "1.8")
-- Could NOT find Doxygen (missing: DOXYGEN_EXECUTABLE) 
-- 
-- Rtosc Build Configuration
-- =========================
-- 
-- PkgConfig enabled  -- package found
-- Liblo enabled      -- package found
-- JACK disabled      -- package NOT found
-- NTK disabled       -- package NOT found
-- Perf Test(s) disabled
-- FLTK disabled      -- package NOT found
-- Checking Include Path
-- Checking Library Path
-- Could NOT find ZLIB (missing: ZLIB_LIBRARIES ZLIB_INCLUDE_DIRS) 
-- ALSA not found
-- Could NOT find OpenGL (missing: OPENGL_opengl_LIBRARY OPENGL_glx_LIBRARY OPENGL_INCLUDE_DIR) 
-- Could NOT find FLTK (missing: FLTK_LIBRARIES FLTK_INCLUDE_DIR FLTK_FLUID_EXECUTABLE) 
-- Could NOT find OpenGL (missing: OPENGL_opengl_LIBRARY OPENGL_glx_LIBRARY OPENGL_INCLUDE_DIR) 
-- Could NOT find CxxTest (missing: CXXTEST_INCLUDE_DIR CXXTEST_PYTHON_TESTGEN_EXECUTABLE) 
Looking For pkg config modules
-- Checking for module 'jack'
--   No package 'jack' found
-- Checking for module 'portaudio-2.0>=19'
--   No package 'portaudio-2.0' found
-- Checking for module 'ntk'
--   No package 'ntk' found
-- Checking for module 'ntk_images'
--   No package 'ntk_images' found
-- Checking for module 'portaudio-2.0>=19'
--   No package 'portaudio-2.0' found
-- Checking for one of the modules 'lash-1.0'
-- Checking for one of the modules 'dssi>=0.9.0'
-- Compiling with liblo
-- Compiling with x86 opcode support
-- Building for Release, flags: -std=c++11 -Wno-unused-parameter -O3 -ffast-math -fomit-frame-pointer -msse -msse2 -mfpmath=sse
-- Include what you use: disabled (IncludeWhatYouUse=OFF)
-- 
-- ZynAddSubFX Build Configuration
-- ===============================
-- 
-- Building on a 'Linux' System
-- PkgConfig -- found
-- zlib      -- NOT found
-- mxml      -- found
-- fftw3     -- found
-- liblo     -- found
-- x11       -- NOT found
-- xpm       -- NOT found
-- fltk      -- NOT found
-- ntk       -- NOT found
-- x11       -- NOT found
-- OSS       -- found
-- ALSA      -- NOT found
-- JACK      -- NOT found
-- PA        -- NOT found
-- Lash      -- NOT found
-- DSSI      -- NOT found
-- CxxTest   -- NOT found
-- Lash      -- NOT enabled
-- DSSI      -- NOT enabled
-- tests     -- NOT enabled
-- ALSA      -- NOT enabled
-- JACK      -- NOT enabled
-- OSS       -- enabled
-- PA        -- NOT enabled
-- c++ async -- usable
-- Link libraries: ZLIB_LIBRARY-NOTFOUND  mxml;pthread lo;pthread 
-- Could NOT find Doxygen (missing: DOXYGEN_EXECUTABLE) 
-- Bash completion script for zynaddsubfx will be installed to /usr/share/bash-completion/completions or fallback to /usr/local/share/bash-completion/completions if unwritable.
CMake Error: The following variables are used in this project, but they are set to NOTFOUND.
Please set them or make sure they are set and tested correctly in the CMake files:
ZLIB_INCLUDE_DIR (ADVANCED)
   used as include directory in directory /home/osboxes/zynaddsubfx/src
   used as include directory in directory /home/osboxes/zynaddsubfx/src
   used as include directory in directory /home/osboxes/zynaddsubfx/src
   used as include directory in directory /home/osboxes/zynaddsubfx/src
   used as include directory in directory /home/osboxes/zynaddsubfx/src
   used as include directory in directory /home/osboxes/zynaddsubfx/src
   used as include directory in directory /home/osboxes/zynaddsubfx/src
   used as include directory in directory 

Ouput continues the same.

I suppose that the following line indicates an issue with Findzlib.cmake.

The following variables are used in this project, but they are set to NOTFOUND.
Please set them or make sure they are set and tested correctly in the CMake files:
ZLIB_INCLUDE_DIR (ADVANCED)

I believe the problem is with the name of the file, as I've found here: https://cmake.org/cmake/help/v3.0/command/find_package.html.

P.S. I am confident that I can make a pull request to solve this issue.

Thanks, Maria

[fundamental]

I get the following output when compiling zynaddsubfx without zlib installed.

Shouldn't this output be expected on a system without zlib installed? zlib is a required dependency, so if it is not available, then the configuration phase should fail.