vcpkg_install: detect lack of Git

The vcpkg_install batch file depends on the availability of a
working Git on the CMD path. This may not be present if the user
has selected the 'bash only' option during Git-for-Windows install.

Detect and tell the user about their lack of a working Git in the CMD
window.

Fixes #2348.
A separate PR git-for-windows/build-extra#258
now highlights the recommended path setting during install.

Signed-off-by: Philip Oakley <philipoakley@iee.email>

compat/vcbuild/vcpkg_install.bat

@@ -36,6 +36,13 @@ REM ================================================================
 
 	dir vcpkg\vcpkg.exe >nul 2>nul && GOTO :install_libraries
 
+	git.exe version 2>nul
+	IF ERRORLEVEL 1 (
+	echo "***"
+	echo "Git not found. Please adjust your CMD path or Git install option."
+	echo "***"
+	EXIT /B 1 )
+
 	echo Fetching vcpkg in %cwd%vcpkg
 	git.exe clone https://github.com/Microsoft/vcpkg vcpkg
 	IF ERRORLEVEL 1 ( EXIT /B 1 )