[vala: 5/7] posix: add feature test macro _GNU_SOURCE for exp10(3) and exp10f(3)

[Rico Tzschichholz <ricotz src gnome org>]

commit e5b136a18394b7ccf8fe85df97eea154852d0f09
Author: Dr. Michael Lauer <mickey vanille-media de>
Date:   Mon Feb 26 17:11:50 2018 +0100

    posix: add feature test macro _GNU_SOURCE for exp10(3) and exp10f(3)
    
    https://bugzilla.gnome.org/show_bug.cgi?id=614788

 vapi/posix.vapi |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/vapi/posix.vapi b/vapi/posix.vapi
index 8eb239f..e944542 100644
--- a/vapi/posix.vapi
+++ b/vapi/posix.vapi
@@ -534,9 +534,9 @@ namespace Posix {
        public double modf (double x, out double iptr);
        [CCode (cheader_filename = "math.h")]
        public float modff (float x, out float iptr);
-       [CCode (cheader_filename = "math.h")]
+       [CCode (cheader_filename = "math.h", feature_test_macro = "_GNU_SOURCE")]
        public double exp10 (double x);
-       [CCode (cheader_filename = "math.h")]
+       [CCode (cheader_filename = "math.h", feature_test_macro = "_GNU_SOURCE")]
        public float exp10f (float x);
        [CCode (cheader_filename = "math.h", feature_test_macro = "_GNU_SOURCE")]
        public double pow10 (double x);