[PATCH v2] configure: add support for pseudo-"in source tree" builds

[Daniel P . Berrangé]

Meson requires the build dir to be separate from the source tree. Many
people are used to just running "./configure && make" though and the
meson conversion breaks that.

This introduces some backcompat support to make it appear as if an
"in source tree" build is being done, but with the the results in the
"build/" directory. This allows "./configure && make" to work as it
did historically, albeit with the output binaries staying under build/.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
---

This is a simple integration of Eric's proposal from

  https://lists.gnu.org/archive/html/qemu-devel/2020-03/msg07290.html

with a bit of configure magic. It is enough to enable

   ./configure
   make
   make check

I've not tested it beyond that. Note it blows away the "build/"
dir each time ./configure is run so it is pristine each time.

We could optionally symlink binaries from build/ into $PWD
if people think that is important, eg by changing GNUmakefile
to have:

recurse: all
        for bin in `find build -maxdepth 1 -type f -executable | grep -v -E 
'(ninjatool|config.status)'`; \
        do \
          ln -f -s $$bin . ; \
        done

and some cleanup logic to purge the symlinks for "make clean"

This goes on top of Paolo's most recent meson port v175 posting,
or whatever number it is upto now :-)

In v2:

 - Use a marker file so we don't blow away a build/ dir
   we didn't create
 - Silence the distclean rule
 - Fix broken use of error_exit that's not defined yet
 - Add comment to GNUmakefile

 .gitignore |  2 ++
 configure  | 48 +++++++++++++++++++++++++++++++++++++++++++++---
 2 files changed, 47 insertions(+), 3 deletions(-)

diff --git a/.gitignore b/.gitignore
index 92311284ef..4ccb9ed975 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,5 @@
+/GNUmakefile
+/build/
 /.doctrees
 /config-devices.*
 /config-all-devices.*
diff --git a/configure b/configure
index cc5f58f31a..cdffe221c7 100755
--- a/configure
+++ b/configure
@@ -11,6 +11,51 @@ unset CLICOLOR_FORCE GREP_OPTIONS
 # Don't allow CCACHE, if present, to use cached results of compile tests!
 export CCACHE_RECACHE=yes
 
+# make source path absolute
+source_path=$(cd "$(dirname -- "$0")"; pwd)
+
+if test "$PWD" == "$source_path"
+then
+    echo "Using './build' as the directory for build output"
+
+    MARKER=build/auto-created-by-configure
+
+    if test -e build
+    then
+       if test -f $MARKER
+       then
+          rm -rf build
+       else
+           echo "ERROR: ./build dir already exists and was not previously 
created by configure"
+           exit 1
+       fi
+    fi
+
+    mkdir build
+    touch $MARKER
+
+    cat > GNUmakefile <<EOF
+# This file is auto-generated by configure to support in-source tree
+# 'make' command invokation
+
+ifeq (\$(MAKECMDGOALS),)
+recurse: all
+endif
+
+.NOTPARALLEL: %
+%: force
+       @echo 'changing dir to build for \$(MAKE) "\$(MAKECMDGOALS)"...'
+       @\$(MAKE) -C build -f Makefile \$(MAKECMDGOALS)
+       @if test "\$(MAKECMDGOALS)" = "distclean" && test -e 
build/auto-created-by-configure ; then rm -rf build GNUmakefile ; fi
+force: ;
+.PHONY: force
+GNUmakefile: ;
+
+EOF
+    cd build
+    exec $source_path/configure "$@"
+fi
+
 # Temporary directory used for files created while
 # configure runs. Since it is in the build directory
 # we can safely blow away any previous version of it
@@ -297,9 +342,6 @@ ld_has() {
     $ld --help 2>/dev/null | grep ".$1" >/dev/null 2>&1
 }
 
-# make source path absolute
-source_path=$(cd "$(dirname -- "$0")"; pwd)
-
 if printf %s\\n "$source_path" "$PWD" | grep -q "[[:space:]:]";
 then
   error_exit "main directory cannot contain spaces nor colons"
-- 
2.26.2