Sample code:

#include <stdio.h>
#include <stdlib.h>

extern void foo() __attribute__((weak));
int main() {
  if (foo) foo();
  printf("this is a test\n");
}

Compile command:

gcc -H -v sample.c 

Compile options:

-H tells you that the right include files (in /usr/local/) are used.
Official document:

Print the name of each header file used, in addition to other normal
activities. Each name is indented to show how deep in the ‘#include’
stack it is. Precompiled header files are also printed, even if they
are found to be invalid; an invalid precompiled header file is printed
with ‘…x’ and a valid one with ‘…!’ .

-v (or even maybe -Wl,-v) tells you that the right libraries (in /usr/local/) are used.
Official document:

Print (on standard error output) the commands executed to run the
stages of compilation. Also print the version number of the compiler
driver program and of the preprocessor and the compiler proper.

Sample output:

…………………….
/usr/include/x86_64-linux-gnu/bits/waitflags.h
/usr/include/x86_64-linux-gnu/bits/waitstatus.h
/usr/include/x86_64-linux-gnu/gnu/stubs-64.h
/usr/include/x86_64-linux-gnu/gnu/stubs.h COLLECT_GCC_OPTIONS=’-o’
‘hello’ ‘-H’ ‘-v’ ‘-mtune=generic’ ‘-march=x86-64’ as -v –64 -o
/tmp/ccwD1DYA.o /tmp/cc8CU0IV.s GNU assembler version 2.24
(x86_64-linux-gnu) using BFD version (GNU Binutils for Ubuntu) 2.24
COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.8/:/usr/lib/gcc/x86_64-linux-gnu/4.8/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.8/:/usr/lib/gcc/x86_64-linux-gnu/
LIBRARY_PATH=/lib/x86_64-linux-gnu/:/lib/../lib/:/usr/lib/gcc/x86_64-linux-gnu/4.8/:/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../lib/:/lib/x86_64-linux-gnu/:/lib/../lib/:/usr/lib/x86_64-linux-gnu/:/usr/lib/../lib/:/lib/:./:/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../:/lib/:/usr/lib/
…………..

Ref:

1. https://gcc.gnu.org/onlinedocs/
2. http://stackoverflow.com/questions/28563263/c-compiling-errors-on-debian