Corrections

[neomafo88]

1. Makefile
   a. Add -lm to host_ldflags
   b. Remove -fuse-ld=gold from LDFLAGS
2. guint id is unused, and can be omitted
   a. https://github.com/oleavr/android-inject-custom/blob/master/inject.c#L17
   b. https://github.com/oleavr/android-inject-custom/blob/master/inject.c#L36
3. agent.so path is hardcoded https://github.com/oleavr/android-inject-custom/blob/master/inject.c#L12