Fix bashism
Reported upstream by email
--- a/configure.ac
+++ b/configure.ac
@@ -16,11 +16,11 @@ fpm2_CFLAGS="${x11_CFLAGS} ${libxml2_CFLAGS} ${nettle_CFLAGS} "
 fpm2_LIBS="${x11_LIBS} ${libxml2_LIBS} ${nettle_LIBS} "
 
 PKG_CHECK_MODULES([gtk3], [gtk+-3.0])
-fpm2_CFLAGS+="${gtk3_CFLAGS} "
-fpm2_LIBS+="${gtk3_LIBS} "
+fpm2_CFLAGS="${fpm2_CFLAGS} ${gtk3_CFLAGS} "
+fpm2_LIBS="${fpm2_LIBS} ${gtk3_LIBS} "
 
 # Get rid of deprecated GTK3 mess
-fpm2_CFLAGS+="-Wno-deprecated-declarations"
+fpm2_CFLAGS="${fpm2_CFLAGS} -Wno-deprecated-declarations"
 
 AC_SUBST(fpm2_CFLAGS)
 AC_SUBST(fpm2_LIBS)