diff --git a/.travis.yml b/.travis.yml
index 22a5e07c0..1cb7a184a 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -8,4 +8,4 @@ os:
   - linux
   - osx
 
-script: ./autogen.sh && ./configure && make
+script: ./autogen.sh && ./configure CFLAGS="-O2 -fno-common" && make
diff --git a/CRT.h b/CRT.h
index 06a5fccbc..6d8ccdd08 100644
--- a/CRT.h
+++ b/CRT.h
@@ -146,7 +146,7 @@ extern const char **CRT_treeStr;
 
 extern int CRT_delay;
 
-int* CRT_colors;
+extern int* CRT_colors;
 
 extern int CRT_colorSchemes[LAST_COLORSCHEME][LAST_COLORELEMENT];
 
@@ -154,13 +154,13 @@ extern int CRT_scrollHAmount;
 
 extern int CRT_scrollWheelVAmount;
 
-char* CRT_termType;
+extern char* CRT_termType;
 
 // TODO move color scheme to Settings, perhaps?
 
 extern int CRT_colorScheme;
 
-void *backtraceArray[128];
+extern void *backtraceArray[128];
 
 #if HAVE_SETUID_ENABLED
 
diff --git a/linux/LinuxProcess.h b/linux/LinuxProcess.h
index 6ccb65a6a..0b64eeec6 100644
--- a/linux/LinuxProcess.h
+++ b/linux/LinuxProcess.h
@@ -187,7 +187,7 @@ typedef struct LinuxProcess_ {
 #endif
 
 
-long long btime; /* semi-global */
+extern long long btime; /* semi-global */
 
 extern ProcessFieldData Process_fields[];