Add explicit include-dir key to manifest

[LKedward]

A minimal set of changes towards improved support for include/#include statements (related: #358).

• Proposes a new manifest key include-dir in the [library] table which allows specifying a directory to pass to the compiler as an include directory (i.e. -Iinclude).
  • This allows included .f90 files to be stored separately to compilable sources so that fpm won't try to compile them.
• The default value of include-dir is "include" which is ignored if the directory doesn't exist.
• A package contains a library if it contains either the source-dir or the include-dir or both
  • This caters for 'header-only' libraries (useful for defining interfaces, preprocesser macros and inlinable routines)

Not addressed in this PR (#358):

• Tracking module dependencies in include files
• Detecting changes in include files for incremental compilation

[awvwgk]

Nice, this allows to use fpm as C/C++ package manager by exporting headers!

I suggest to allow include-dir to take a string or an array of strings (similar to build.link) in case multiple include directories are present.

[milancurcic]

Is this PR expected to also work for including pre-built module files?

Specifically, I'm trying to use this to build a program with NetCDF as a dependency. A system-provided netcdf.mod is in /usr/include. Here's my fpm.toml:

name = "umwm"

[library]
source-dir = "staging"
include-dir = "/usr/include"

[build]
link = "netcdff"

But I get the error during fpm build:

...
Unable to find source for module dependency: "netcdf" used by "././staging/umwm_io.f90"
ERROR STOP 1
...

Is the problem perhaps that fpm tries to look for the source file that provides the netcdf module?

Compiler options are:

 <INFO>COMPILER OPTIONS:   -Wall -Wextra -Wimplicit-interface -fPIC -fmax-errors=1 -g -fbounds-check -fcheck-array-temporaries -fbacktrace -fcoarray=single  -J build/gfortran_debug/umwm -I build/gfortran_debug/umwm

So it doesn't look like the path I provided in include-dir was passed to compiler options.

Or am I not setting it up correctly?

[LKedward]

Is this PR expected to also work for including pre-built module files?

No, this PR isn't targeting this use-case specifically, but you are right that this is only limited by the fact that fpm tries to identify a source file all modules used. We could loosen this check to only print a warning if the module source file isn't contained within the project and then your example will work. I think your use case should be discussed separately to this PR; using external mod files isn't ideal for fpm IMO, but I guess it can't be avoided for packages like netcdf.

Looks like I also just need to update the compiler options output to run after processing the manifest.

[milancurcic]

Sounds good, in that case, let's just add a note to the manifest reference that this for the time being won't work for pre-built modules.

[awvwgk]

A very useful addition, only one minor whitespace nitpick in the manifest reference from my side.

[LKedward]

Thanks for reviewing, I will now merge.