Fix gyp builds with cflags.

Don't merge yet, work still needs to be done to get shared libaries generating correctly. To test shared libraries do
```
gyp --depth=. --generator_output=output -f make -D "library=shared_library"
```

I get the following error
```
make: Entering directory `/home/larz/Programming/c/libsphia/output'
  CC(target) out/Release/obj.target/str-ends-with/deps/str-ends-with/str-ends-with.o
  AR(target) out/Release/obj.target/deps/str-ends-with/libstr-ends-with.a
  CC(target) out/Release/obj.target/sphia/src/clear.o
  CC(target) out/Release/obj.target/sphia/src/count.o
  CC(target) out/Release/obj.target/sphia/src/error.o
  CC(target) out/Release/obj.target/sphia/src/free.o
  CC(target) out/Release/obj.target/sphia/src/get.o
  CC(target) out/Release/obj.target/sphia/src/new.o
  CC(target) out/Release/obj.target/sphia/src/purge.o
  CC(target) out/Release/obj.target/sphia/src/rm.o
  CC(target) out/Release/obj.target/sphia/src/set.o
  SOLINK(target) out/Release/obj.target/libsphia.so
/usr/bin/ld: out/Release/obj.target/deps/str-ends-with/../../str-ends-with/deps/str-ends-with/str-ends-with.o: relocation R_X86_64_PC32 against undefined symbol `strlen@@GLIBC_2.2.5' can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld: final link failed: Bad value
collect2: error: ld returned 1 exit status
make: *** [out/Release/obj.target/libsphia.so] Error 1
make: Leaving directory `/home/larz/Programming/c/libsphia/output'
```