Here are mine:

• -Wextra and -Wall: essential.
• -Wfloat-equal: useful because usually testing floating-point numbers for equality is bad.
• -Wundef: warn if an uninitialized identifier is evaluated in an #if directive.
• -Wshadow: warn whenever a local variable shadows another local variable, parameter or global variable or whenever a built-in function is shadowed.
• -Wpointer-arith: warn if anything depends upon the size of a function or of void.
• -Wcast-align: warn whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if a char * is cast to an int * on machines where integers can only be accessed at two- or four-byte boundaries.
• -Wstrict-prototypes: warn if a function is declared or defined without specifying the argument types.
• -Wstrict-overflow=5: warns about cases where the compiler optimizes based on the assumption that signed overflow does not occur. (The value 5 may be too strict, see the manual page.)
• -Wwrite-strings: give string constants the type const char[length] so that copying the address of one into a non-const char * pointer will get a warning.
• -Waggregate-return: warn if any functions that return structures or unions are defined or called.
• -Wcast-qual: warn whenever a pointer is cast to remove a type qualifier from the target type^*.
• -Wswitch-default: warn whenever a switch statement does not have a default case^*.
• -Wswitch-enum: warn whenever a switch statement has an index of enumerated type and lacks a case for one or more of the named codes of that enumeration^*.
• -Wconversion: warn for implicit conversions that may alter a value^*.
• -Wunreachable-code: warn if the compiler detects that code will never be executed^*.

Those marked ^* sometimes give too many spurious warnings, so I use them on as-needed basis.

It's right there in the man page of gcc (called with man gcc on unix/linux or you can find it via Google):

-I dir

Add the directory dir to the list of directories to be searched for header files. Directories named by -I are searched before the standard system include directories. If the directory dir is a standard system include directory, the option is ignored to ensure that the default search order for system directories and the special treatment of system headers are not defeated . If dir begins with "=", then the "=" will be replaced by the sysroot prefix; see --sysroot and -isysroot.

(The exact text and semantics may differ between versions of gcc)

Alternatively there is also the gcc documentation online: http://gcc.gnu.org/onlinedocs/ The option for version 4.8.2, as an example, can be found here: http://gcc.gnu.org/onlinedocs/gcc-4.8.2/gcc/Directory-Options.html#Directory-Options

You don't need any environment variables, just pass in right cflags and ldflags that SDL2 wants you to use:

gcc main.c `pkg-config --cflags sdl2` -o main `pkg-config --libs sdl2`

or either

gcc main.c `sdl2-config --cflags` -o main `sdl2-config --libs`

Remember: CFLAGS come before LDFLAGS, and LDFLAGS (and library specification with -l) comes last.

SDL2 comes with sdl2-config script preinstalled. You will need to set your PATH to the directory where it resides to call it successfully:

export PATH=/opt/SDL2/bin:$PATH

If you will run every of *-config commands directly, you will see that they just output right cflags and ldflags for you. That's because the libraries that employ these scripts usually bigger than to specify single -I/-L argument, and it is not portable to specify single -I/-L arguments for them, because number of such arguments can be increased in future.

And you should not install every package in it's own directory. Install everything into /usr/local for example, then you will not need even to specify anything (most distros point you to /usr/local automatically).