[cmake] Simplify DetectPlatform and normalize platform names (#4183)

Since 3.25 introduced `LINUX` and `BSD`, use them, and standardize other platforms to that style

Signed-off-by: crueter <crueter@eden-emu.dev>
Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/4183
Reviewed-by: Samuel <lizzie@eden-emu.dev>
Reviewed-by: CamilleLaVey <camillelavey99@gmail.com>

CMakeModules/find/FindOpus.cmake

@@ -13,7 +13,7 @@ find_package_handle_standard_args(Opus
     VERSION_VAR OPUS_VERSION
 )
 
-if (PLATFORM_MSYS)
+if (MSYS2)
     FixMsysPath(PkgConfig::OPUS)
 endif()
 

CMakeModules/find/Findenet.cmake

@@ -14,7 +14,7 @@ find_package_handle_standard_args(enet
     VERSION_VAR ENET_VERSION
 )
 
-if (PLATFORM_MSYS)
+if (MSYS2)
     FixMsysPath(PkgConfig::ENET)
 endif()
 

CMakeModules/find/Findlibusb.cmake

@@ -14,7 +14,7 @@ find_package_handle_standard_args(libusb
     VERSION_VAR LIBUSB_VERSION
 )
 
-if (PLATFORM_MSYS)
+if (MSYS2)
     FixMsysPath(PkgConfig::LIBUSB)
 endif()
 

CMakeModules/find/Findlz4.cmake

@@ -13,7 +13,7 @@ else()
     find_package(PkgConfig QUIET)
     pkg_search_module(LZ4 QUIET IMPORTED_TARGET liblz4)
 
-    if (PLATFORM_MSYS)
+    if (MSYS2)
         FixMsysPath(PkgConfig::LZ4)
     endif()