Regressions caused by libgap.pc PR

[fingolfin]

Unfortunately PR #5080 by @dimpase (UPDATE: together with PR #5106 by me -- which also had a bug completely of its own) introduced a few regressions:

1. weirdly, the makemanuals ... CI test is now failing
2. every time one enters make now, even if nothing else is to be done, one now gets this message:

$ make
Package libgap was not found in the pkg-config search path.
Perhaps you should add the directory containing `libgap.pc'
to the PKG_CONFIG_PATH environment variable
No package 'libgap' found

I am guessing problem 2 is because the (shell PKG_CONFIG_PATH=$(DESTDIR)$(libdir)/pkgconfig pkg-config --modversion libgap) is always executed. I think to fix this one needs to replace the ifeq by a shell if.

No idea right now what causes 1, but it might in fact be related

[fingolfin]

Aha, I think I see what went wrong with 1. I'll submit a PR to deal with both

[dimpase]

mea cupla...

[fingolfin]

Nah, no worries. Tests didn't catch it, and >60% of the problem were my fault anyway :-/