Allow to pass in custom CFLAGS to gcc

[Jonny Heggheim]

Hi, my name is Jonny Heggheim[1], I am the maintainer of the openjfx
package in Fedora[2].

Could you help me with this issue?
https://bugzilla.redhat.com/show_bug.cgi?id=1472613

The standard build for Fedora is to build with symbols and then strip
out the debug symbols into a separate -debug package.

Would it be possible to modify the build so it is possible to pass in
custom CFLAGS?

The default flags for Fedora 26 builds on x86_64 is:
$ rpm --eval '%{optflags}'
-O2 -g -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2
-fexceptions -fstack-protector-strong --param=ssp-buffer-size=4
-grecord-gcc-switches -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1
-m64 -mtune=generic

Sincerely Jonny Heggheim

[1] https://fedoraproject.org/wiki/User:Jonny
[2] https://apps.fedoraproject.org/packages/openjfx