ARROW-6237: [R] Allow compilation flags to be passed for R package with ARROW_R_CXXFLAGS

[wesm]

For example:

export ARROW_R_CXXFLAGS="-fno-omit-frame-pointer"

(this flag is needed to record performance data on Linux with perf)

I also amended the README.Rmd to say to use $R_LD_LIBRARY_PATH instead of $LD_LIBRARY_PATH for custom install location

[nealrichardson]

A couple of notes but otherwise LGTM

[nealrichardson]

Could you remove this added comment output? It's not helpful IMO. (If you regenerate this with make doc, it doesn't appear because the arrow package is already loaded before this code chunk is evaluated.)

[wesm]

I see, will do

[nealrichardson]

Add a note to the README about this feature?

[wesm]

Will do

[nealrichardson]

Thanks, LGTM.

[codecov-io]

Codecov Report

Merging #5088 into master will decrease coverage by 12.37%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #5088       +/-   ##
===========================================
- Coverage   87.63%   75.25%   -12.38%     
===========================================
  Files        1013       57      -956     
  Lines      144891     3617   -141274     
  Branches     1418        0     -1418     
===========================================
- Hits       126971     2722   -124249     
+ Misses      17558      895    -16663     
+ Partials      362        0      -362

Impacted Files	Coverage Δ
python/pyarrow/ipc.pxi
cpp/src/arrow/csv/chunker-test.cc
cpp/src/parquet/column_page.h
cpp/src/parquet/bloom_filter-test.cc
cpp/src/arrow/array/builder_decimal.cc
cpp/src/plasma/client.cc
cpp/src/arrow/io/test-common.h
cpp/src/arrow/util/int-util-test.cc
rust/parquet/src/arrow/converter.rs
cpp/src/arrow/python/io.cc
... and 946 more

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