[gcab] build: Explicitly enable C99 support

[Philip Withnall <pwithnall src gnome org>]

commit b95728249789fb786dbb68a56d696b92e432aa25
Author: Philip Withnall <withnall endlessm com>
Date:   Fri Dec 1 17:14:14 2017 +0000

    build: Explicitly enable C99 support
    
    We use C99 declarations in loops in gcab-cabinet.c. For the benefit of
    older compilers (Debian Jessie, I’m looking at you), explicitly enable
    C99 support.
    
    Signed-off-by: Philip Withnall <withnall endlessm com>
    
    https://bugzilla.gnome.org/show_bug.cgi?id=791094

 configure.ac |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index c841712..b8b8a91 100644
--- a/configure.ac
+++ b/configure.ac
@@ -11,6 +11,7 @@ AM_MAINTAINER_MODE([enable])
 AM_SILENT_RULES([yes])
 
 AC_PROG_CC
+AC_PROG_CC_C99
 AM_PROG_CC_C_O
 AC_PROG_INSTALL
 LT_INIT([win32-dll])