Build should support custom CFLAGS

[brson]

Right now CFLAGS is always overridden by the Makefiles, so turning off optimizations and turning on debug info is difficult.

[burg]

For which submodules is this a problem?

[brson]

I've had this problem with libcss. I assume that part of the problem is https://github.com/mozilla/servo/blob/master/Makefile.in#L102, which looks like it unconditionally assigns both CFLAGS and RUSTFLAGS.

[Meta-phaze]

Testing:

Case 1:
- In build/src/libcss: make clean
- In build/: CFLAGS=-g make
- confirm build output: cc -g -I/Users/Doug/servo/src/libcss/src ....

Case 2:
- In build/src/libcss: make clean
- In build/: CFLAGS=-O2 make
- confirm build output: cc -O2 -I/Users/Doug/servo/src/libcss/src ....

Case 3:
- In build/src/libcss: make clean
- In build/: make
- confirm build output: cc -I/Users/Doug/servo/src/libcss/src ....

• Did not test all subcomponents, only libcss
• Did not adjust RUSTFLAGS, is this needed?

[metajack]

That patch looks ok, but it seems like we have now lost -O2 in the default case.

[Meta-phaze]

If that is desirable it is easy to add. I will do that right now.

[Meta-phaze]

Testing:

Case 1:

• clean in libcss
• in build/: make
• confirm: cc -O2 -I/Users/Doug/servo/src/libcss/src ...

Case 2:

• clean in libcss
• in build/: CFLAGS=-g make
• confirm: cc -g -I/Users/Doug/servo/src/libcss/src ...

[metajack]

Thanks.