Use SFML_ROOT to find SFML installation

This is line with modern CMake convention where -D<PackageName>_ROOT
is used to locate a given package's installation.

https://cmake.org/cmake/help/latest/variable/PackageName_ROOT.html#variable:%3CPackageName%3E_ROOT

tools/BuildMacOS.sh

@@ -371,7 +371,7 @@ build_csfml () # $1: 'clang' => clang & libc++
           -D "CMAKE_OSX_DEPLOYMENT_TARGET:STRING=$target" \
           -D "CMAKE_OSX_SYSROOT:STRING=$sdk" \
           -D "BUILD_DOC:BOOL=$doc" \
-          -D "SFML_DIR:PATH=$sfml_package1070/Frameworks/SFML.framework/Versions/$gittag/Resources/CMake" \
+          -D "SFML_ROOT:PATH=$sfml_package1070/Frameworks/SFML.framework/Versions/$gittag/Resources/CMake" \
           "$codedir"
     assert $? "CMake failed"
 

tools/nuget/build.linux.sh

@@ -113,7 +113,7 @@ CSFMLLibDir="$(realpath lib)" # The directory that contains the final CSFML libr
 
 cmake -E env LDFLAGS="-z origin" \
     cmake \
-    "-DSFML_DIR=$SFMLBuiltDir" \
+    "-DSFML_ROOT=$SFMLBuiltDir" \
     '-DBUILD_SHARED_LIBS=1' \
     '-DCMAKE_BUILD_TYPE=Release' \
     "-DCMAKE_LIBRARY_OUTPUT_DIRECTORY=$CSFMLLibDir" \

tools/nuget/build.macos.sh

@@ -133,7 +133,7 @@ CSFMLLibDir="$(realpath lib)" # The directory that contains the final CSFML libr
 
 cmake -E env \
     cmake -G "Unix Makefiles" \
-          -D "SFML_DIR=$SFMLBuiltDir" \
+          -D "SFML_ROOT=$SFMLBuiltDir" \
           -D 'BUILD_SHARED_LIBS=ON' \
           -D 'CMAKE_BUILD_TYPE=Release' \
           -D "CMAKE_OSX_ARCHITECTURES=$ARCHITECTURE" \

tools/nuget/build.win.ps1

@@ -141,7 +141,7 @@ New-Item -ItemType Directory lib > $null
 $CSFMLLibDir = (Get-Item lib).FullName; # The directory where the final CSFML dlls are located
 
 cmake `
-    "-DSFML_DIR=$SFMLBuiltDir" `
+    "-DSFML_ROOT=$SFMLBuiltDir" `
     '-DCSFML_LINK_SFML_STATICALLY=1' `
     "-DCMAKE_LIBRARY_PATH=$SFMLExtLibs" `
     `