configure: Support the extended pkgconf syntax

Make possible to require a combination of
packages and/or pin specific versions.
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

configure

@@ -960,8 +960,8 @@ check_pkg_config(){
     pkg_cflags=$($pkg_config --cflags $pkg_config_flags $pkg)
     pkg_libs=$($pkg_config --libs $pkg_config_flags $pkg)
     check_func_headers "$headers" "$funcs" $pkg_cflags $pkg_libs "$@" &&
-        set_safe ${pkg}_cflags $pkg_cflags   &&
-        set_safe ${pkg}_libs   $pkg_libs
+        set_safe "${pkg}_cflags" $pkg_cflags   &&
+        set_safe "${pkg}_libs"   $pkg_libs
 }
 
 check_exec(){
@@ -1057,8 +1057,8 @@ require2(){
 require_pkg_config(){
     pkg="$1"
     check_pkg_config "$@" || die "ERROR: $pkg not found"
-    add_cflags    $(get_safe ${pkg}_cflags)
-    add_extralibs $(get_safe ${pkg}_libs)
+    add_cflags    $(get_safe "${pkg}_cflags")
+    add_extralibs $(get_safe "${pkg}_libs")
 }
 
 hostcc_e(){