OS specific instructions in CMAKE: How to?

Use

if (WIN32)
    #do something
endif (WIN32)

or

if (UNIX)
    #do something
endif (UNIX)

or

if (MSVC)
    #do something
endif (MSVC)

or similar

see CMake Useful Variables and CMake Checking Platform

In General

You can detect and specify variables for several operating systems like that:

Detect Microsoft Windows

if(WIN32)
    # for Windows operating system in general
endif()

Or:

if(MSVC OR MSYS OR MINGW)
    # for detecting Windows compilers
endif()

Detect Apple MacOS

if(APPLE)
    # for MacOS X or iOS, watchOS, tvOS (since 3.10.3)
endif()

Detect Unix and Linux

if(UNIX AND NOT APPLE)
    # for Linux, BSD, Solaris, Minix
endif()

Your specific linker issue

To solve your issue with the Windows-specific wsock32 library, just remove it from other systems, like that:

if(WIN32)
    target_link_libraries(${PROJECT_NAME} bioutils wsock32)
else
    target_link_libraries(${PROJECT_NAME} bioutils)
endif()

You have some special words from CMAKE, take a look:

if(${CMAKE_SYSTEM_NAME} STREQUAL "Linux")
    // do something for Linux
else
    // do something for other OS

Given this is such a common issue, geronto-posting:

    if(UNIX AND NOT APPLE)
        set(LINUX TRUE)
    endif()

    # if(NOT LINUX) should work, too, if you need that
    if(LINUX) 
        message(STATUS ">>> Linux")
        # linux stuff here
    else()
        message(STATUS ">>> Not Linux")
        # stuff that should happen not on Linux 
    endif()

CMake boolean logic docs

CMake platform names, etc.