cmake(windows): set correct path to the system Git config

Currently, when Git for Windows is built with CMake, the system Git config is
expected in a different location than when building via `make`: the former
expects it to be in `<runtime-prefix>/mingw64/etc/gitconfig`, the latter in
`<runtime-prefix>/etc/gitconfig`.

Because of this, things like `git clone` do not work correctly (because cURL is
no longer able to find its certificate bundle that it needs to validate HTTPS
certificates). See the full bug report and discussion here:
git-for-windows#3071 (comment).

This commit aligns the CMake-based build by mimicking what is already done in
`config.mak.uname`.

This closes git-for-windows#3071.

Signed-off-by: Dennis Ameling <dennis@dennisameling.com>

contrib/buildsystems/CMakeLists.txt

@@ -204,8 +204,6 @@ list(APPEND compat_SOURCES sha1dc_git.c sha1dc/sha1.c sha1dc/ubc_check.c block-s
 
 
 add_compile_definitions(PAGER_ENV="LESS=FRX LV=-c"
-			ETC_GITATTRIBUTES="etc/gitattributes"
-			ETC_GITCONFIG="etc/gitconfig"
 			GIT_EXEC_PATH="libexec/git-core"
 			GIT_LOCALE_PATH="share/locale"
 			GIT_MAN_PATH="share/man"
@@ -220,10 +218,15 @@ add_compile_definitions(PAGER_ENV="LESS=FRX LV=-c"
 
 if(WIN32)
 	set(FALLBACK_RUNTIME_PREFIX /mingw64)
-	add_compile_definitions(FALLBACK_RUNTIME_PREFIX="${FALLBACK_RUNTIME_PREFIX}")
+	# Move system config into top-level /etc/
+	add_compile_definitions(FALLBACK_RUNTIME_PREFIX="${FALLBACK_RUNTIME_PREFIX}"
+		ETC_GITATTRIBUTES="../etc/gitattributes"
+		ETC_GITCONFIG="../etc/gitconfig")
 else()
 	set(FALLBACK_RUNTIME_PREFIX /home/$ENV{USER})
-	add_compile_definitions(FALLBACK_RUNTIME_PREFIX="${FALLBACK_RUNTIME_PREFIX}")
+	add_compile_definitions(FALLBACK_RUNTIME_PREFIX="${FALLBACK_RUNTIME_PREFIX}"
+		ETC_GITATTRIBUTES="etc/gitattributes"
+		ETC_GITCONFIG="etc/gitconfig")
 endif()