BSD: Only use BSD as variable for target BSD system

Instead of having multiple boolean variables, we only have one
string variable: BSD with the name of the target BSD system.

Relates: #23853

Help/manual/cmake-variables.7.rst

@@ -324,8 +324,6 @@ Variables that Describe the System
    /variable/CMAKE_SYSTEM_PROCESSOR
    /variable/CMAKE_SYSTEM_VERSION
    /variable/CYGWIN
-   /variable/DRAGONFLYBSD
-   /variable/FREEBSD
    /variable/GHSMULTI
    /variable/IOS
    /variable/LINUX
@@ -344,8 +342,6 @@ Variables that Describe the System
    /variable/MSVC_TOOLSET_VERSION
    /variable/MSVC_VERSION
    /variable/MSYS
-   /variable/NETBSD
-   /variable/OPENBSD
    /variable/UNIX
    /variable/WIN32
    /variable/WINCE

Help/release/dev/BSD.rst

@@ -0,0 +1,5 @@
+Variables
+---------
+
+* The :variable:`BSD` variable is set to a string value when the target system is BSD.
+  This value can be one of the following: DragonFlyBSD, FreeBSD, OpenBSD, or NetBSD.

Help/release/dev/bsd-variables.rst

@@ -1,9 +0,0 @@
-bsd-variables
--------------
-
-* The :variable:`BSD` variable is set to true when the target system
-  is BSD (DragonFlyBSD, FreeBSD, OpenBSD, or NetBSD)
-* The :variable:`DRAGONFLYBSD` variable is set to true when the target system is DragonFlyBSD.
-* The :variable:`FREEBSD` variable is set to true when the target system is FreeBSD.
-* The :variable:`NETBSD` variable is set to true when the target system is NetBSD.
-* The :variable:`OPENBSD` variable is set to true when the target system is OpenBSD.

Help/variable/BSD.rst

@@ -3,4 +3,5 @@ BSD
 
 .. versionadded:: 3.25
 
-Set to true when the target system is BSD (DragonFlyBSD, FreeBSD, OpenBSD, or NetBSD)
+Set to a string value when the target system is BSD. This value can be one of
+the following: DragonFlyBSD, FreeBSD, OpenBSD, or NetBSD.

Help/variable/DRAGONFLYBSD.rst

@@ -1,6 +0,0 @@
-DRAGONFLYBSD
-------------
-
-.. versionadded:: 3.25
-
-Set to true when the target system is DragonFlyBSD.

Help/variable/FREEBSD.rst

@@ -1,6 +0,0 @@
-FREEBSD
--------
-
-.. versionadded:: 3.25
-
-Set to true when the target system is FreeBSD.

Help/variable/NETBSD.rst

@@ -1,6 +0,0 @@
-NETBSD
-------
-
-.. versionadded:: 3.25
-
-Set to true when the target system is NetBSD.

Help/variable/OPENBSD.rst

@@ -1,6 +0,0 @@
-OPENBSD
--------
-
-.. versionadded:: 3.25
-
-Set to true when the target system is OpenBSD.

Modules/Platform/DragonFly.cmake

@@ -3,8 +3,7 @@
 # see http://archive.netbsd.se/?ml=dfbsd-users&a=2007-07&m=4678361
 
 include(Platform/FreeBSD)
-unset(FREEBSD)
-set(DRAGONFLYBSD 1)
+set(BSD "DragonFlyBSD")
 
 # DragonFly BSD requires -z origin to enable $ORIGIN expansion in RPATH.
 # This is not required for FreeBSD since 10.2-RELEASE.

Modules/Platform/FreeBSD.cmake

@@ -1,5 +1,4 @@
-set(FREEBSD 1)
-set(BSD 1)
+set(BSD "FreeBSD")
 set(CMAKE_DL_LIBS "")
 set(CMAKE_C_COMPILE_OPTIONS_PIC "-fPIC")
 set(CMAKE_C_COMPILE_OPTIONS_PIE "-fPIE")

Modules/Platform/NetBSD.cmake

@@ -1,5 +1,4 @@
-set(NETBSD 1)
-set(BSD 1)
+set(BSD "NetBSD")
 set(CMAKE_DL_LIBS "")
 set(CMAKE_C_COMPILE_OPTIONS_PIC "-fPIC")
 set(CMAKE_C_COMPILE_OPTIONS_PIE "-fPIE")

Modules/Platform/OpenBSD.cmake

@@ -1,6 +1,5 @@
 include(Platform/NetBSD)
-unset(NETBSD)
-set(OPENBSD 1)
+set(BSD "OpenBSD")
 
 # On OpenBSD, the compile time linker does not share it's configuration with
 # the runtime linker.  This will extract the library search paths from the