travis: add build with iconv enabled

... to verify it builds and works fine.

Ref: https://curl.haxx.se/mail/lib-2017-09/0031.html

Closes curl#1872

.travis-iconv-env.sh

@@ -0,0 +1 @@
+export CPPFLAGS="-DCURL_DOES_CONVERSIONS -DHAVE_ICONV -DCURL_ICONV_CODESET_OF_HOST='\"ISO8859-1\"'"

.travis.yml

@@ -51,6 +51,10 @@ matrix:
           compiler: clang
           dist: trusty
           env: T=debug
+        - os: linux
+          compiler: gcc
+          dist: trusty
+          env: T=iconv
         - os: osx
           compiler: gcc
           env: T=debug
@@ -170,6 +174,14 @@ script:
              make && make examples
              make test-nonflaky
         fi
+    - |
+        set -eo pipefail
+        if [ "$T" = "iconv" ]; then
+             source .travis-iconv-env.sh
+             ./configure --enable-debug --enable-werror $C
+             make && make examples
+             make TFLAGS=-n test-nonflaky
+        fi
     - |
         set -eo pipefail
         if [ "$T" = "cmake" ]; then