use os specific prefix

unable to replace files in /usr on osx

Author: glensc

Diff for: .travis.yml

@@ -1,7 +1,7 @@
 language: c
 
 os:
- - linux
+# - linux
  - osx
 
 before_install:
@@ -15,14 +15,17 @@ before_install:
 script:
  # abort on any command failure
  - set -e
+   # travis debug
+ - which file || echo no
 
  # build and install file
  - git clone --depth=100 git://github.com/file/file.git
  - cd file
  - test -z "$commit" || git checkout $commit
  - git log -1
  - autoreconf -f -i
- - ./configure --prefix=/usr
+ - case "$TRAVIS_OS_NAME" in osx) prefix=/usr/local;; linux) prefix=/usr;; esac
+ - ./configure --prefix=$prefix
  - make -j4
    # remove system file package first
    # see https://github.com/file/file-tests/issues/3