Use explicit INSTALL_COMMAND to suppress DESTDIR (#21)

The DESTDIR environment variable is used on UNIX systems during an
invocation of 'make install' to specify an alternate directory prefix
for installation paths. Since this isn't a cross-platform mechanism,
the CMAKE_INSTALL_PREFIX CMake variable can also be used.

Unfortunately, if both of these are specified, they will both be
applied. This isn't typically a problem, but can cause problems for the
installation phase of ExternalProject_Add if the external project is
built during an invocation of 'make install' with DESTDIR set.

Since the CMAKE_INSTALL_PREFIX method is employed by the call to
ExternalProject_Add instead of DESTDIR, we should specifically suppress
DESTDIR if it is set so that it doesn't interfere with the installation
of the external project to the staging directory.

Signed-off-by: Scott K Logan <logans@cottsay.net>

CMakeLists.txt

@@ -52,6 +52,12 @@ macro(build_uncrustify)
       -Wno-dev
     PATCH_COMMAND
       ${CMAKE_COMMAND} -E chdir <SOURCE_DIR> git checkout -q . && ${CMAKE_COMMAND} -E chdir <SOURCE_DIR> git apply -p1 --ignore-space-change --whitespace=nowarn ${CMAKE_CURRENT_SOURCE_DIR}/install-patch.diff
+    # Workaround for https://gitlab.kitware.com/cmake/cmake/-/issues/18165,
+    # which is exacerbated by https://gitlab.kitware.com/cmake/cmake/-/issues/16419.
+    # We're configuring the project to use CMAKE_INSTALL_PREFIX, so we never want
+    # DESTDIR to affect it.
+    INSTALL_COMMAND
+      ${CMAKE_COMMAND} -E env DESTDIR= ${CMAKE_COMMAND} --install .
   )
 
   # The external project will install to the build folder, but we'll install that on make install.