$ git show 4f219ba08 CMakeLists.txt
commit 4f219ba08744095a7f545ccc7cc76a3b01c15279
Author: Pol Marcet Sardà <polmarcetsarda@gmail.com>
Date:   Mon Jan 21 23:35:42 2019 +0100

    - Wadseeker and Doomseeker can now be built independently on non-Windows platforms. (0003280)
    -- Doomseeker can specify an external dependency to Wadseeker if FORCE_INTERNAL_WADSEEKER option 
is turned off (it's on by default).
    -- Wadseeker also now can be built from the source file generated by "makesourcepackages.sh"
    
- Now Wadseeker exports a Config.cmake (with other required files) to make it easy to import into other 
cmake projects.

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 9147b933..4e6635c7 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -78,6 +78,7 @@ if(WIN32)
 else()
        option(FORCE_INTERNAL_ZLIB "Use internal zlib" OFF)
        option(FORCE_INTERNAL_BZIP2 "Use internal bzip2" OFF)
+       option(FORCE_INTERNAL_WADSEEKER "Use internal wadseeker" ON)

        include(${CMAKE_SOURCE_DIR}/cmake/Modules/Macros.cmake)

@@ -87,6 +88,9 @@ else()
        if(NOT FORCE_INTERNAL_BZIP2)
                find_package_BZip2()
        endif()
+       if(NOT FORCE_INTERNAL_WADSEEKER)
+               find_library_wadseeker()
+       endif()
 endif()

 if( ZLIB_FOUND AND NOT FORCE_INTERNAL_ZLIB )
@@ -115,6 +119,16 @@ else( BZIP2_FOUND AND NOT FORCE_INTERNAL_BZIP2 )
        set( BZIP2_LIBRARY bz2 )
 endif()

+if ( wadseeker_FOUND AND NOT FORCE_INTERNAL_WADSEEKER )
+       message( STATUS "Using system Wadseeker library" )
+else( wadseeker_FOUND AND NOT FORCE_INTERNAL_WADSEEKER )
+       message( STATUS "Using internal Wadseeker library" )
+       set( WADSEEKER_INTERNAL YES )
+       add_subdirectory( src/wadseeker )
+       set( WADSEEKER_INCLUDE_DIRS "${CMAKE_CURRENT_SOURCE_DIR}/src/wadseeker" )
+       set( WADSEEKER_INCLUDE_DIR "${WADSEEKER_INCLUDE_DIRS}" )
+endif()
+
 if(MSVC AND NOT MSVC90 AND NOT MSVC12)
        message(WARNING
                "Full automatic packaging is only supported for Microsoft Visual C++ 2008 and 2013. 
"
@@ -134,7 +148,6 @@ add_subdirectory(tools/updaterevision)

 add_subdirectory(src/core)
 add_subdirectory(src/plugins)
-add_subdirectory(src/wadseeker)

 message(STATUS "'INSTALL' will dump files to \"${CMAKE_INSTALL_PREFIX}\"")

$ rm CMakeCache.txt
$ cmake -DFORCE_INTERNAL_WADSEEKER=OFF /home/wub/.local/src/doomseeker/doomseeker
-- The C compiler identification is GNU 11.2.0
-- The CXX compiler identification is GNU 11.2.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Found ZLIB: /usr/lib/x86_64-linux-gnu/libz.so (found version "1.2.11")
-- Found BZip2: /usr/lib/x86_64-linux-gnu/libbz2.so (found version "1.0.8")
-- Looking for BZ2_bzCompressInit
-- Looking for BZ2_bzCompressInit - found
-- Could NOT find wadseeker (missing: wadseeker_DIR)
-- Using system zlib
-- Using system bzip2 library
-- Using internal Wadseeker library
-- Looking for pthread.h
-- Looking for pthread.h - found
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD - Failed
-- Looking for pthread_create in pthreads
-- Looking for pthread_create in pthreads - not found
-- Looking for pthread_create in pthread
-- Looking for pthread_create in pthread - found
-- Found Threads: TRUE
-- Setting up plugin chocolatedoom. Source dir:
    /home/wub/.local/src/doomseeker/doomseeker/src/plugins/chocolate-doom
--     chocolatedoom provides translations
-- Setting up plugin odamex. Source dir:
    /home/wub/.local/src/doomseeker/doomseeker/src/plugins/odamex
--     odamex provides translations
-- Setting up plugin srb2. Source dir:
    /home/wub/.local/src/doomseeker/doomseeker/src/plugins/srb2
--     srb2 provides translations
-- Setting up plugin zandronum. Source dir:
    /home/wub/.local/src/doomseeker/doomseeker/src/plugins/zandronum
--     zandronum provides translations
-- Setting up plugin zandronumq. Source dir:
    /home/wub/.local/src/doomseeker/doomseeker/src/plugins/zandronumq
--     zandronumq provides translations
-- Setting up plugin turok2ex. Source dir:
    /home/wub/.local/src/doomseeker/doomseeker/src/plugins/turok2ex
--     turok2ex provides translations
-- 'INSTALL' will dump files to "/usr/local"
-- Configuring done
-- Generating done
-- Build files have been written to: /tmp/doomseeker

Quote from CMakeLists.txt

if ( wadseeker_FOUND AND NOT FORCE_INTERNAL_WADSEEKER )
        message( STATUS "Using system Wadseeker library" )
else( wadseeker_FOUND AND NOT FORCE_INTERNAL_WADSEEKER )
        message( STATUS "Using internal Wadseeker library" )
        // ...
endif()

Quote from CMake 3.22.1 Documentation
Per legacy, the else() and endif() commands admit an optional <condition> argument. If used, it must be a verbatim repeat of the argument of the opening if command.