Don't build with -Werror

Unless you're testing with ALL compiler versions, including the unreleased ones. You're not testing the GCC version released one year ago.

```
gcc -g -O3 -feliminate-unused-debug-types -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector --param=ssp-buffer-size=32 -Wformat -Wformat-security -Wl,--copy-dt-needed-entries -m64 -fasynchronous-unwind-tables -Wp,-D_REENTRANT -ftree-loop-distribute-patterns -Wl,-z -Wl,now -Wl,-z -Wl,relro -fno-semantic-interposition -ffat-lto-objects -fno-signed-zeros -fno-trapping-math -fassociative-math -Wl,-sort-common -Wl,--enable-new-dtags -mtune=skylake -Wjump-misses-init -Wstrict-prototypes -Wunsuffixed-float-constants -fstack-protector-strong -O2 -D_FORTIFY_SOURCE=2 -UDEBUG -DNDEBUG -ffunction-sections -fdata-sections -Wall -Wextra -Winit-self -Wpointer-arith -Wreturn-type -Waddress -Wsequence-point -Wformat-security -Wmissing-include-dirs -Wfloat-equal -Wundef -Wshadow -Wcast-align -Wconversion -Wredundant-decls -DITT_ARCH_IA64 -Wjump-misses-init -Wstrict-prototypes -Wunsuffixed-float-constants -fstack-protector-strong -O2 -D_FORTIFY_SOURCE=2 -UDEBUG -DNDEBUG -ffunction-sections -fdata-sections -Wall -Wextra -Winit-self -Wpointer-arith -Wreturn-type -Waddress -Wsequence-point -Wformat-security -Wmissing-include-dirs -Wfloat-equal -Wundef -Wshadow -Wcast-align -Wconversion -Wredundant-decls -DITT_ARCH_IA64 -Wjump-misses-init -Wstrict-prototypes -Wunsuffixed-float-constants -fstack-protector-strong -O2 -D_FORTIFY_SOURCE=2 -UDEBUG -DNDEBUG -ffunction-sections -fdata-sections -Wall -Wextra -Winit-self -Wpointer-arith -Wreturn-type -Waddress -Wsequence-point -Wformat-security -Wmissing-include-dirs -Wfloat-equal -Wundef -Wshadow -Wcast-align -Wconversion -Wredundant-decls -DITT_ARCH_IA64 -Wjump-misses-init -Wstrict-prototypes -Wunsuffixed-float-constants -fstack-protector-strong -O2 -D_FORTIFY_SOURCE=2 -UDEBUG -DNDEBUG -ffunction-sections -fdata-sections -Wall -Wextra -Winit-self -Wpointer-arith -Wreturn-type -Waddress -Wsequence-point -Wformat-security -Wmissing-include-dirs -Wfloat-equal -Wundef -Wshadow -Wcast-align -Wconversion -Wredundant-decls -DITT_ARCH_IA64 -Wjump-misses-init -Wstrict-prototypes -Wunsuffixed-float-constants -fstack-protector-strong -O2 -D_FORTIFY_SOURCE=2 -UDEBUG -DNDEBUG -ffunction-sections -fdata-sections -Wall -Wextra -Winit-self -Wpointer-arith -Wreturn-type -Waddress -Wsequence-point -Wformat-security -Wmissing-include-dirs -Wfloat-equal -Wundef -Wshadow -Wcast-align -Wconversion -Wredundant-decls -DITT_ARCH_IA64 -DDISABLE_TRACE -fPIC -DSE_SIM -Werror -g -I ../../../include -fPIC -Wno-strict-prototypes -c -o ittnotify_static.o ittnotify_static.c
ittnotify_static.c: In function ‘__itt_init_ittlib’:
ittnotify_static.c:978:36: error: this statement may fall through [-Werror=implicit-fallthrough=]
groups = __itt_group_legacy;
~~~~~~~^~~~~~~~~~~~~~~~~~~~
ittnotify_static.c:979:25: note: here
case 1:
^~~~
cc1: all warnings being treated as errors
```