travis: do not use OVERRIDE_CC or OVERRIDE_CXX if empty

Fixes the macOS builds where OVERRIDE_CC and OVERRIDE_CXX are not set.

Reported-by: Jay Satiro
Fixes #4659
Closes #4661
Closes #4664

diff --git a/.travis.yml b/.travis.yml
index c157f7c87a04bd7b7bcc9ec59750d6c406ccfc78..94e13bfeb808d92dcc349d3ddc2bf69e786d85c4 100644 (file)
--- a/.travis.yml
+++ b/.travis.yml
@@ -439,8 +439,8 @@ matrix:
                       - zlib1g-dev
 
 before_install:
-    - export "${OVERRIDE_CC}"
-    - export "${OVERRIDE_CXX}"
+  - export "${OVERRIDE_CC-blank=}"
+  - export "${OVERRIDE_CXX-blank=}"
 
 install:
   - if [ "$T" = "coverage" ]; then pip2 install --user cpp-coveralls; fi