build: Add check for 'git' on Windows and macOS deps scripts

Since git is actually not a dependency, we need to do some checks.

See: https://gitlab.gnome.org/Infrastructure/gimp-web-devel/-/issues/54

build/macos/1_build-deps-macports.sh

@@ -14,6 +14,8 @@ elif [ $(basename "$PWD") = 'macos' ]; then
   cd ../../..
 fi
 if [ -z "$GITLAB_CI" ]; then
+  which git >/dev/null 2>&1 || { printf '\033[31m(ERROR)\033[0m: git is required for this script. Please, read: https://developer.gimp.org/core/setup/git/#source-from-git\n'; exit 1; }
+
   GIT_DEPTH='1'
 
   PARENT_DIR='/..'

build/windows/1_build-deps-msys2.ps1

@@ -14,6 +14,8 @@ elseif (Test-Path 1_build-deps-msys2.ps1 -Type Leaf)
   }
 if (-not $GITLAB_CI)
   {
+    if (-not (Get-Command "git" -ErrorAction SilentlyContinue)) { Write-Host '(ERROR): git is required for this script. Please, read: https://developer.gimp.org/core/setup/git/#source-from-git' -ForegroundColor Red; exit 1 }
+
     $GIT_DEPTH = '1'
 
     $PARENT_DIR = '\..'