ARROW-7804: [C++][R] Compile error on macOS 10.11

This fixes the uninitialized variables and updates the `macos-r-autobrew` crossbow job to test on macOS 10.11, which is what CRAN uses.

Closes #6388 from nealrichardson/el-capitan and squashes the following commits:

8a95569 <Neal Richardson> Comments to explain this archaic mess
8f821c8 <Neal Richardson> Revert "Correct hack (revert me)"
50d9835 <Neal Richardson> Remove checks that require LaTeX
6d3afb3 <Neal Richardson> Fix hack for ancient xcode
dea999f <Neal Richardson> Try older xcode
58b9a1d <Neal Richardson> Correct hack (revert me)
81b1a79 <Neal Richardson> Revert "Hack to test cpp change (revert me)"
5ab13be <Neal Richardson> Hack to test cpp change (revert me)
7e24a84 <Neal Richardson> Attempt to fix compilation error
6bd6566 <Neal Richardson> No TeX
e979b57 <Neal Richardson> Use old xcode on Travis to match CRAN's macOS 10.11

Authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>

cpp/src/parquet/stream_writer.h

@@ -230,10 +230,10 @@ class PARQUET_EXPORT StreamWriter {
 };
 
 struct PARQUET_EXPORT EndRowType {};
-constexpr EndRowType EndRow;
+constexpr EndRowType EndRow = {};
 
 struct PARQUET_EXPORT EndRowGroupType {};
-constexpr EndRowGroupType EndRowGroup;
+constexpr EndRowGroupType EndRowGroup = {};
 
 PARQUET_EXPORT
 StreamWriter& operator<<(StreamWriter&, EndRowType);

dev/tasks/homebrew-formulae/travis.osx.r.yml

@@ -15,10 +15,13 @@
 # limitations under the License.
 
 os: osx
+# CRAN builds on macOS 10.11, so make sure that's what we're testing
+osx_image: xcode7.3
 language: r
 cache: packages
 r: {{ r_version }}
 fortran: false
+latex: false
 cran: https://mac.r-project.org
 
 # don't build twice
@@ -37,12 +40,16 @@ before_install:
 - git -C arrow fetch -t {{ arrow.remote }} {{ arrow.branch }}
 - if [ $CROSSBOW_USE_COMMIT_ID = true ]; then git -C arrow checkout {{ arrow.head }}; else git -C arrow checkout FETCH_HEAD; fi
 - cd arrow/r
-# This script used to be inside r/tools/ but was moved
+# Put the formula inside r/ so that it's included in the package build
 - cp ../dev/tasks/homebrew-formulae/autobrew/apache-arrow.rb tools/apache-arrow.rb
+before_script:
+# Hack for old macOS (10.11)
+# See https://travis-ci.community/t/library-developer-commandlinetools-sdks-macosx-sdk-missing-on-el-capitain/3213/3
+- sed -i.bak 's/-isysroot /-I/g' $(R RHOME)/etc/Makeconf
 script:
-- travis_wait R CMD build .
+- travis_wait R CMD build --no-build-vignettes .
 # Note that this is not --as-cran. CRAN doesn't do macOS checks --as-cran
-- travis_wait R CMD check arrow_*.tar.gz --install-args="--build"
+- travis_wait R CMD check arrow_*.tar.gz --no-manual --ignore-vignettes --install-args="--build"
 after_failure:
 # If there's a build failure, it's probably in this log
 - cat arrow.Rcheck/00install.out