[configure] Clean up CPPFLAGS after preprocessing flags tests

Otherwise, plug-ins compiled in internal mode that do some additional
C-related checks are not 

configure.in

@@ -533,8 +533,11 @@ AC_MSG_RESULT(Default preprocessor is '$FRAMAC_DEFAULT_CPP'.)
 
 DEFAULT_CPP_SUPPORTED_ARCH_OPTS=
 
+# Store original value of CPPFLAGS before doing tests
+OLD_CPPFLAGS=$CPPFLAGS
+
 rm -f conftest.i
-CPPFLAGS="-m32"
+CPPFLAGS="$OLD_CPPFLAGS -m32"
 AC_PREPROC_IFELSE(
   [AC_LANG_SOURCE([/* Check if preprocessor supports option -m32 */])],
   [if test -e conftest.i; then
@@ -542,7 +545,7 @@ AC_PREPROC_IFELSE(
   fi], [])
 
 rm -f conftest.i
-CPPFLAGS="-m64"
+CPPFLAGS="$OLD_CPPFLAGS -m64"
 AC_PREPROC_IFELSE(
   [AC_LANG_SOURCE([/* Check if preprocessor supports option -m64 */])],
   [if test -e conftest.i; then
@@ -550,13 +553,16 @@ AC_PREPROC_IFELSE(
   fi], [])
 
 rm -f conftest.i
-CPPFLAGS="-m16"
+CPPFLAGS="$OLD_CPPFLAGS -m16"
 AC_PREPROC_IFELSE(
   [AC_LANG_SOURCE([/* Check if preprocessor supports option -m16 */])],
   [if test -e conftest.i; then
     DEFAULT_CPP_SUPPORTED_ARCH_OPTS+='\"-m16\"; ';
   fi], [])
 
+# revert CPPFLAGS to original value
+CPPFLAGS=$OLD_CPPFLAGS
+
 AC_MSG_RESULT(Default preprocessor supported architecture-related options: $DEFAULT_CPP_SUPPORTED_ARCH_OPTS)