Fix SSL CA cert issue on CentOS

Fix "Problem with the SSL CA cert" issue @
https://github.com/google/edash-packager/issues/6. On Ubuntu, CURL_CA_BUNDLE
is located at "/etc/ssl/certs/ca-certificates.crt" while on CentOs, it is
located at "/etc/ssl/certs/ca-bundle.crt".

With this change, GYP scans the common appearing locations for SSL CA cert.

Change-Id: I838bae3954ec51f1430c900d1e82596d45a4db84

packager/third_party/curl/config/linux/curl_config.h

@@ -2,7 +2,7 @@
 /* lib/curl_config.h.in.  Generated from configure.ac by autoheader.  */
 
 /* Location of default ca bundle */
-#define CURL_CA_BUNDLE "/etc/ssl/certs/ca-certificates.crt"
+/* CURL_CA_BUNDLE is defined by GYP */
 
 /* Location of default ca path */
 /* #undef CURL_CA_PATH */

packager/third_party/curl/curl.gyp

@@ -5,6 +5,23 @@
 # https://developers.google.com/open-source/licenses/bsd
 
 {
+  'variables': {
+    # Scan ca bundle in its common appearing locations.
+    'curl_ca_bundle%':
+      '<!(for path in /etc/pki/tls/certs/ca-bundle.crt       \
+                      /etc/ssl/cert.pem                      \
+                      /etc/ssl/certs/ca-bundle.crt           \
+                      /etc/ssl/certs/ca-certificates.crt     \
+                      /usr/local/share/certs/ca-root.crt     \
+                      /usr/share/ssl/certs/ca-bundle.crt; do \
+            if test -f "$path"; then ca="$path"; break; fi;  \
+          done;                                              \
+          if [ -z "$ca" ]; then                              \
+            echo "Failed to locate SSL CA cert."; exit 1;    \
+          else                                               \
+            echo "$ca";                                      \
+          fi)',
+  },
   'targets': [
     {
       'target_name': 'curl_config',
@@ -18,6 +35,7 @@
           'USE_IPV6',
           'USE_OPENSSL'
           'USE_SSLEAY',
+          'CURL_CA_BUNDLE="<(curl_ca_bundle)"',
         ],
         'include_dirs': [
           'config',