[r1.2] Fix typo: CMAKE_COMPILER_IS_GNUCXX

[njzjz]

Cherry picked from #1038.

---

The flag won't work without language specified.
See https://cmake.org/cmake/help/v3.4/variable/CMAKE_COMPILER_IS_GNULANG.html

(cherry picked from commit 8bbe565)

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (r1.2@69df1d2). Click here to learn what that means.
The diff coverage is n/a.

❗ Current head 8bbe565 differs from pull request most recent head 09a6ac1. Consider uploading reports for the commit 09a6ac1 to get more accurate results

@@           Coverage Diff           @@
##             r1.2    #1040   +/-   ##
=======================================
  Coverage        ?   82.90%           
=======================================
  Files           ?      120           
  Lines           ?    10127           
  Branches        ?        0           
=======================================
  Hits            ?     8396           
  Misses          ?     1731           
  Partials        ?        0           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 69df1d2...09a6ac1. Read the comment docs.