Handle quoted assignment arguments with whitespace

If we are going to set e.g. CXXFLAGS to more than one value, the
quotes must be preserved at the same time as the list is laundered
(for autotools-variables)

configure

@@ -280,7 +280,10 @@ done
 shift $((OPTIND-1))
 
 # remove Autotools-specific variables
-ENVVARS=${@/ACLOCAL_*=*/}
+for a in "$@"; do
+  a="${a/ACLOCAL_*=*/}"
+  [ -n "$a" ] && ENVVARS="$ENVVARS \"${a/\"/\\\"}\""
+done
 
 # pass everything on to CMake
 CMDLINE="env ${ENVVARS} ${CMAKE_COMMAND} \"${srcdir}\" \"-DCMAKE_INSTALL_PREFIX=$prefix\" -DCMAKE_BUILD_TYPE=${buildtype}${pch_use}${silent_rules}${debug_loc} ${FEATURES}"